AIIMS 2004 Biology Question Paper with Answer and Solution

(A) Bacterial flagella are composed of a protein called $flagellin$. This protein polymerizes to form the helical structure of the flagellum,which is responsible for bacterial motility.

(B) In $Ulothrix$,the life cycle is haplontic. The zygote undergoes a period of dormancy and forms a thick-walled zygospore. When favourable conditions return,the zygospore undergoes meiosis $(reduction\, division)$ to produce $4-16$ haploid meiospores. Each of these meiospores germinates to form a new $Ulothrix$ filament.

(A) Mosses (Bryophytes) and ferns (Pteridophytes) are found in moist and shady habitats because they require water for the process of fertilization.
In these plants,the male gametes (antherozoids) are flagellated and motile,and they need a film of water to swim to the female gamete (egg) to achieve fertilization.
Therefore,the presence of water is an essential external factor for their sexual reproduction.

(C) Companion cells are specialized parenchyma cells that are closely associated with sieve tube elements in the phloem of angiosperms.
They are connected to sieve tube elements through plasmodesmata.
Companion cells maintain the pressure gradient in the sieve tubes and assist in the loading and unloading of sugars (sucrose) into the sieve elements.

(B) Cork cells,also known as phellem,are formed during secondary growth in plants.
These cells have thick walls that are impregnated with a waxy,hydrophobic substance called $Suberin$.
$Suberin$ deposition makes the cell walls impervious to water and gases,which protects the inner tissues of the plant from desiccation and mechanical injury.

(D) Lenticels are small,lens-shaped openings or pores found in the periderm of woody stems and roots.
These structures consist of loosely arranged parenchyma cells that allow for the exchange of gases between the internal tissues of the plant and the external atmosphere.
Since the periderm is generally impermeable to gases and stomata are absent in these woody organs,lenticels serve as the primary site for gaseous exchange and limited transpiration.

(D) $DNA$ is present in mitochondria and chloroplasts.
These organelles are known as semi-autonomous organelles because they contain their own circular $DNA$ and ribosomes,which allow them to synthesize some of their own proteins.
This $DNA$ is referred to as extra-nuclear genetic material.

(A) Sugarcane ($Saccharum$ $officinarum$) belongs to the family $Poaceae$ (also known as $Gramineae$),which is a monocotyledonous family.
In monocots,the guard cells of the stomata are typically dumb-bell shaped,whereas in dicots,they are usually kidney or bean-shaped.
Therefore,the correct characteristic of the sugarcane plant is that it possesses dumb-bell shaped guard cells.

(B) $O_2$ is not liberated in bacteria during the process of photosynthesis. Their photosynthesis is non-oxygenic. This is because these bacteria use $H_2S$ (hydrogen sulfide) instead of $H_2O$ (water) as a hydrogen donor during the light-dependent reactions.

(C) Apical dominance is maintained by the presence of $Auxin$ (specifically $Indole$ $acetic$ $acid$) produced in the apical bud,which inhibits the growth of axillary buds.
When the apical bud is removed (pruning),the source of $Auxin$ is eliminated.
This removal allows the axillary buds to grow,as they become sensitized to $Cytokinin$,which promotes lateral bud development and branching.

(B) The $SA$ node (Sinoatrial node) is located in the right atrium of the heart and is responsible for initiating the electrical impulses that cause the heart to contract. Because it sets the rhythm of the heartbeat,it is commonly referred to as the natural pacemaker of the heart.
- Malleus is a bone in the middle ear,while the anvil is the common name for the incus bone; they are distinct.
- Leucocytes are white blood cells (a general category),while lymphocytes are a specific type of white blood cell.
- Haemophilia is a genetic bleeding disorder,whereas blood cancer is known as leukaemia.

(C) When irregularity in the heart rhythm occurs,the normal heart rhythm can be restored and maintained by surgically implanting an artificial pacemaker. This device sends out small electrical impulses to stimulate the heart muscle to contract at a regular rate.

(D) The transmission of a nerve impulse across a synapse is unidirectional because neurotransmitters are stored in synaptic vesicles located only at the axon terminals (presynaptic knobs).
When an action potential reaches the axon terminal,it triggers the release of these neurotransmitters into the synaptic cleft.
Dendrites possess receptors for these neurotransmitters but do not contain synaptic vesicles to release them.
Therefore,the impulse can only travel from the axon of one neuron to the dendrite or cell body of the next neuron,ensuring unidirectional flow.

(A) Red algae,specifically members of the order $Corallinales$,are known as coralline algae.
These algae secrete and deposit calcium carbonate $(CaCO_3)$ in their cell walls,which makes them hard and stony.
These calcareous deposits contribute significantly to the formation of coral reefs and limestone deposits in marine environments.
Therefore,the Reason correctly explains why red algae contribute to the production of coral reefs.

(A) The leech (Annelida),cockroach (Arthropoda),and scorpion (Arthropoda) all belong to the phyla of invertebrates that possess a ventral nerve cord.
$1$. Leech belongs to Phylum Annelida,which is characterized by a ventral nerve cord.
$2$. Cockroach and scorpion belong to Phylum Arthropoda,which also possess a ventral nerve cord.
$3$. Nephridia are specific to Annelids.
$4$. Cephalization is not a defining common feature for all three.
$5$. Antennae are present in insects (cockroach) but absent in leeches and scorpions.

(C) Ginger $(Zingiber officinale)$ is a classic example of a rhizome,which is a modified underground stem that grows horizontally (prostrate) under the soil surface.
Therefore,the Assertion is correct.
Regarding the Reason,shoot growth is indeed influenced by gravity (gravitropism/geotropism). Shoots typically exhibit negative geotropism (growing away from gravity),while roots exhibit positive geotropism.
Since the statement 'Shoot growth is not affected by gravity' is scientifically incorrect,the Reason is false.
Thus,the correct option is $C$.

(C) The Assertion is correct because sulphur deficiency leads to chlorosis (yellowing of leaves) in plants,particularly in younger leaves.
However,the Reason is incorrect because sulphur is not a constituent of chlorophyll. The central atom of the chlorophyll molecule is magnesium $(Mg^{2+})$.
Sulphur is a component of certain amino acids (like cysteine and methionine),vitamins (like biotin and thiamine),and coenzymes,but it is not part of the chlorophyll structure.

(A) The cyclic pathway of photosynthesis is considered the most primitive form,appearing first in some eubacterial species. It is the earliest mechanism for $ATP$ production using light energy.
Non-cyclic photophosphorylation involves the photolysis of water,where water molecules are split into oxygen and hydrogen under the influence of light and chlorophyll. This process is the primary natural source of molecular oxygen in the atmosphere. Therefore,oxygen accumulation began only after the evolution of the non-cyclic pathway. Both statements are correct,and the Reason explains why oxygen accumulation is linked to the evolution of the non-cyclic pathway.

(A) Diabetes insipidus $(DI)$ is a condition characterized by the excretion of large amounts of dilute urine and excessive thirst (polydipsia).
This condition occurs due to the deficiency or lack of response to the antidiuretic hormone $(ADH)$,also known as vasopressin.
$ADH$ is synthesized by the hypothalamus and stored and released by the posterior lobe of the pituitary gland (neurohypophysis).
$ADH$ acts on the kidney tubules (specifically the collecting ducts) to increase water reabsorption.
When $ADH$ is deficient,the kidneys cannot conserve water,leading to excessive urination.
Therefore,both the Assertion and the Reason are correct,and the Reason explains why the lack of $ADH$ leads to the symptoms described in the Assertion.

(A) The enzyme nitrogenase,which is responsible for nitrogen fixation in legume root nodules,is highly sensitive to molecular oxygen $(O_2)$.
To function effectively,it requires an anaerobic environment.
Leghaemoglobin acts as an oxygen scavenger,binding to oxygen and maintaining a low oxygen concentration within the nodules.
Therefore,both the Assertion and the Reason are correct,and the Reason correctly explains why the enzyme functions at a low oxygen concentration.

(B) Potato ($Solanum$ $tuberosum$) is a modification of the stem,while sweet potato ($Ipomoea$ $batatas$) is a modification of the root.
Although both serve the same function of food storage,they originate from different plant parts (stem vs. root).
Organs that have different developmental origins but perform similar functions are called analogous organs.
Therefore,the edible parts of potato and sweet potato are analogous organs.

(D) In the structure of nucleic acid bases,the number of hydrogen bond donor and acceptor sites determines the binding capability.
$Adenine$ $(A)$ typically forms $2$ hydrogen bonds with $Thymine$ $(T)$ in $DNA$.
$Guanine$ $(G)$ forms $3$ hydrogen bonds with $Cytosine$ $(C)$.
Among the options,$Adenine$ is characterized by having two specific hydrogen bonding sites (one donor and one acceptor) that allow it to pair specifically with $Thymine$ via two hydrogen bonds.

(C) After reaching the ovary,the pollen tube enters the ovule through the micropyle.
Inside the embryo sac,the pollen tube releases the two male gametes into the cytoplasm of one of the synergids.
Therefore,the pollen tube typically enters the embryo sac by passing through one of the synergids,guided by the filiform apparatus present in them.

(C) The original chromosome segment order is $A-B-C-D-E$.
In the mutated chromosome,the segment $B-C-D$ has been reversed to $D-C-B$.
This type of chromosomal mutation,where a segment of a chromosome breaks off and is reinserted in the reverse orientation,is known as Inversion.
Therefore,the correct option is $C$.

(D) In the given pedigree,both parents have free earlobes (represented by open symbols),but they have children with attached earlobes (represented by filled symbols).
Since the parents show the dominant trait (free earlobes) and produce offspring with the recessive trait (attached earlobes),both parents must carry the recessive allele.
Therefore,the parents must be heterozygous $(Aa)$ for the trait.
When two heterozygous parents $(Aa \times Aa)$ reproduce,they can produce offspring with the genotype $aa$ (attached earlobes),which explains the presence of affected children in the pedigree.

(B) The cross between $AA BB CC$ and $aa bb cc$ produces an $F_1$ progeny with the genotype $Aa Bb Cc$.
To find the number of different types of gametes produced by an individual with $n$ heterozygous gene pairs,we use the formula $2^n$.
In the genotype $Aa Bb Cc$,there are $3$ heterozygous gene pairs $(n = 3)$.
Therefore,the number of different types of gametes = $2^3 = 2 \times 2 \times 2 = 8$.
The possible gametes are $ABC, ABc, AbC, Abc, aBC, aBc, abC, abc$.

(B) The genetic code is degenerate and specific.
- $UUA$ codes for Leucine.
- $AAA$ codes for Lysine.
- $AUG$ codes for Methionine (it also acts as a start codon).
- $CCC$ codes for Proline.
Therefore,the pair $AAA$-Lysine is correctly matched.

(D) The human genome contains approximately $3.1647$ billion nucleotide base pairs. According to the Human Genome Project,the total number of nitrogenous bases is estimated to be about $3.1$ billion.

(A) Rishikesh is home to the $IDPL$ ($Indian$ $Drugs$ $and$ $Pharmaceuticals$ $Limited$),which is a major public sector undertaking in India. It is primarily known for the large-scale manufacturing of various essential drugs,including antibiotics,vitamins,and other pharmaceutical products.

(B) $SARS$ stands for Severe Acute Respiratory Syndrome.
It is a viral respiratory disease caused by the $SARS-associated$ coronavirus $(SARS-CoV)$,which is a member of the coronavirus family,the same family that includes viruses responsible for the common cold.
It is characterized by high fever,headache,and respiratory symptoms like cough and shortness of breath.

(D) Thalidomide was a drug prescribed in the late $1950s$ and early $1960s$ to pregnant women to treat morning sickness (nausea and vomiting).
It was later discovered to be a potent teratogen,meaning it causes birth defects.
The most characteristic effect of thalidomide exposure during the critical period of embryonic development is phocomelia,which results in severely shortened or underdeveloped limbs (malformed limbs).
Therefore,the correct option is $D$.

(C) The correct answer is $C$.
An allopolyploid is an organism that contains two or more sets of chromosomes derived from different species.
Triticale is the first man-made cereal crop,which was developed by crossing wheat ($Triticum$ $aestivum$) and rye ($Secale$ $cereale$).
It combines the high yield and grain quality of wheat with the hardiness and disease resistance of rye.

(D) The Great Fisher Bank is a large area of the North Sea,located off the coast of Great Britain. It is one of the most important fishing grounds in the world due to the mixing of warm and cold ocean currents,which creates ideal conditions for the growth of plankton,the primary food source for fish.

(C) When domestic sewage rich in organic waste enters a lake,it promotes the growth of microorganisms that decompose the organic matter. This process consumes a large amount of dissolved oxygen $(DO)$ from the water. As the level of $DO$ drops significantly,aquatic animals like fish cannot perform respiration through their gills,leading to their death.

(C) High-altitude birds often act as essential pollinators or seed dispersers for specific plant species in mountain ecosystems.
Orchids,particularly those found in high-altitude regions,often rely on specific bird species for pollination.
If these birds become rare or extinct,the reproductive cycle of these orchids is disrupted,leading to their potential disappearance from the ecosystem.
Therefore,the correct answer is $C$.

(C) The Great Barrier Reef is a large,complex natural unit consisting of biotic components (corals,fish,algae,etc.) interacting with abiotic components (water,sunlight,temperature,nutrients).
Since it represents the interaction of a biological community with its physical environment,it is classified as an ecosystem.

(B) At high altitudes,many plants rely on birds for pollination. Orchids,in particular,often have specialized relationships with specific bird species for pollination. If birds become rare,the pollination of these plants is severely affected,leading to a decline in their population and eventual disappearance.

(A) An endangered species is a species which has been categorized as very likely to become extinct.
$A$. Hornbill (a bird) and Indian Aconite (a medicinal plant) are both listed as endangered species in various conservation assessments.
$B$. Indian peacock is not endangered,and Carrot Grass $(Parthenium)$ is an invasive weed.
$C$. Garden lizard is common,and Mexican poppy is an invasive weed.
$D$. Rhesus monkey is common,and Sal tree is a widely distributed forest tree.

(C) When a species becomes extinct,the unique genetic information it carries is permanently removed from the biosphere. This is known as the loss of its gene pool. While the extinction of a top predator like the Bengal tiger can cause ecological imbalances (such as an increase in prey populations),the most fundamental and irreversible biological consequence is the permanent loss of its genetic diversity,which can never be recovered.

(D) The noise level in a quiet office environment is typically measured at approximately $60 \text{ dB}$.
This level is considered moderate and is consistent with standard office equipment and human conversation levels.

(B) Nitrogen oxides $(NO_x)$ emitted from automobiles and power plants react in the atmosphere to form fine particulate matter and secondary pollutants. These nitrogen oxides,along with sulfur dioxide $(SO_2)$,are primary precursors to acid rain. When these gases are converted into acids (nitric acid and sulfuric acid) and settle on the Earth's surface in the absence of precipitation,it is known as dry acid deposition. Therefore,nitrogen oxides are a significant source of dry acid deposition.

(C) When domestic sewage rich in organic waste enters a lake,it leads to eutrophication.
Microorganisms like bacteria decompose this organic matter,which requires a large amount of dissolved oxygen.
This process significantly reduces the Biological Oxygen Demand $(BOD)$ and depletes the dissolved oxygen levels in the water.
As a result,aquatic organisms,especially fish,die due to the lack of oxygen (asphyxiation).

(B) Minamata disease is a neurological syndrome caused by severe mercury poisoning.
It was first discovered in Minamata City,Japan,in $1956$.
The disease occurred due to the consumption of fish and shellfish contaminated with methylmercury,which was discharged into the Minamata Bay by a local chemical factory.
Mercury is a persistent pollutant that bioaccumulates in the food chain,leading to high concentrations in aquatic organisms.

(D) Birbal Sahni $(1891-1949)$ was a renowned Indian scientist who made significant contributions to the study of fossils of plants. He is widely recognized as the father of Indian Paleobotany. He founded the Birbal Sahni Institute of Palaeosciences in Lucknow.

(C) Sweet clover,when spoilt,contains a compound called dicumarol.
Dicumarol acts as an anticoagulant by inhibiting the synthesis of vitamin $K$-dependent clotting factors in the liver.
As a result,cattle consuming this hay may experience a functional deficiency of vitamin $K$,leading to an inability of the blood to clot properly,which causes prolonged and potentially fatal bleeding.

(A) The correct match is $A$. The National Institute of Virology $(NIV)$ is located in Pune,Maharashtra.
- The National Institute of Communicable Diseases $(NICD)$,now known as the National Centre for Disease Control $(NCDC)$,is located in Delhi.
- The Central Drug Research Institute $(CDRI)$ is located in Lucknow,not Kasauli.
- The National Institute of Nutrition $(NIN)$ is located in Hyderabad,not Mumbai.

(C) Electron beam therapy $(EBT)$ is a form of external beam radiation therapy where high-energy electrons are used to treat cancer.
These electrons are generated by a linear accelerator and are used to treat superficial tumors,such as those found in the skin or near the surface of the body,because they have limited penetration depth.
Therefore,it is primarily used for treating certain types of cancer.

(A) The coconut fruit has a fibrous mesocarp that traps air,making it buoyant. This allows the fruit to float on seawater and travel long distances across oceans. Because the fruit is resistant to saltwater and remains viable for a long time,it can germinate after being washed ashore on distant coastal areas. Thus,the wide distribution of coconut trees in coastal regions is directly due to this mechanism of water dispersal (hydrochory). Therefore,both the Assertion and the Reason are correct,and the Reason is the correct explanation of the Assertion.

(A) The honey bee queen copulates only once in her lifetime during the nuptial flight and stores the received sperms in her spermatheca (seminal receptacle).
She can control the release of sperms from the spermatheca during egg-laying.
If she releases sperms,the eggs are fertilized (developing into females/workers/queens),and if she does not,the eggs remain unfertilized (developing into males/drones).
Therefore,both the Assertion and Reason are correct,and the Reason explains why the queen can lay both types of eggs despite mating only once.

(B) The Assertion is correct because tapeworms $(Taenia)$,roundworms $(Ascaris)$,and pinworms $(Enterobius)$ are well-known endoparasites that inhabit the human intestine.
The Reason is also correct as the consumption of improperly cooked or contaminated food and water is a primary source of transmission for these intestinal parasites.
However,the Reason is not the direct explanation for the Assertion. The Assertion describes the biological classification and habitat of these organisms,while the Reason describes the mode of transmission. Therefore,both statements are true,but the Reason does not explain why they are endoparasites.

(D) The Assertion is incorrect because insects visit flowers primarily to collect nectar,which they later process into honey,and pollen. They do not visit flowers specifically to 'gather honey' as it is a product they make themselves.
The Reason is also incorrect because the primary evolutionary purpose of floral attraction (color,scent,nectar) is to facilitate pollination by attracting pollinators,not to prevent damage to other plant parts.

(A) The Assertion is correct because chimpanzees are indeed the closest living relatives of humans,sharing approximately $98-99\%$ of their $DNA$.
The Reason is also correct. Cytogenetic studies have shown that the banding patterns of chromosomes,particularly in autosomes $3$ and $6$,are remarkably similar between humans and chimpanzees.
This similarity in banding patterns serves as strong evidence for their close evolutionary relationship,thus the Reason correctly explains the Assertion.

(D) The Assertion is incorrect because there is no scientific evidence suggesting that the human gestation period is shortening from an evolutionary perspective. The gestation period is biologically fixed to ensure the complete development of the foetus.
The Reason is correct. $A$ significant evolutionary trend in humans is the development of a larger brain (encephalization),which requires a faster growth rate of the head during the foetal stage to accommodate the increased cranial capacity.
Since the Assertion is false and the Reason is true,the correct option is $D$.

(C) Coacervates are large colloidal aggregates formed due to intermolecular attraction from large organic molecules synthesized abiotically on primitive earth.
They mainly consist of proteins,polysaccharides,and water.
While they were once suggested to be potential precursors of life due to their ability to segregate organic molecules,they do not possess the ability to self-duplicate.
Coacervates lack a lipid outer membrane and genetic material,which are essential for reproduction and self-duplication.
Therefore,the Assertion is considered correct in the context of historical scientific hypotheses,but the Reason is incorrect because coacervates are not self-duplicating.

(A) Natural selection is the process by which individuals with favorable variations are better adapted to their environment,leading to higher survival and reproductive success.
This differential survival and reproduction result in the propagation of advantageous traits within a population.
Adaptive forms of a trait provide a survival advantage,allowing those individuals to reproduce more effectively,thus increasing the frequency of these traits in subsequent generations.
Conversely,less adaptive forms are selected against,leading to their decline or eventual disappearance from the population.
Therefore,the Reason correctly explains the mechanism behind the Assertion.

(D) Amphetamines are stimulants that increase the activity of the $CNS$.
Lysergic acid diethylamide $(LSD)$ is a well-known hallucinogen.
Heroin is a depressant (opioid) that slows down body functions.
Benzodiazepines are sedatives or tranquilizers used for anxiety and sleep disorders,not primarily as pain killers (analgesics). Therefore,the pair 'Benzodiazepine $-$ pain killer' is incorrect as they are not classified as analgesics.

(D) The Assertion is incorrect because a 'Dope test' is specifically used to detect the presence of performance-enhancing drugs in athletes,not to measure blood alcohol levels. Breathalyzers are used for alcohol testing.
The Reason is also incorrect because a person under the influence of alcohol typically exhibits symptoms like loss of inhibition,increased talkativeness,and reduced coordination,rather than feeling tense or less talkative.
Therefore,both the Assertion and the Reason are incorrect.

(D) Interferons are not antibodies; they are a class of glycoproteins known as cytokines produced by cells in response to viral infections,not bacterial infections. Therefore,the Assertion is incorrect.
Interferons do not primarily stimulate inflammation at the site of injury; their main function is to inhibit viral replication and induce an antiviral state in neighboring cells. Therefore,the Reason is also incorrect.
Since both the Assertion and the Reason are false,the correct option is $D$.

(A) Fish meal is a high-protein supplement produced from the non-edible parts of fish,such as fins,tails,and bones,which are byproducts of the fish processing industry.
Because these parts are rich in proteins,minerals,and vitamins,fish meal serves as an excellent nutritional supplement for livestock,including cattle and poultry.
Therefore,the Assertion is correct because fish meal is indeed a rich source of protein,and the Reason correctly explains that it is derived from the non-edible parts of fish.

(A) Cattle breeds can be improved by superovulation and embryo transfer technique,which is known as Multiple Ovulation Embryo Transfer Technology $(MOET)$.
In this method,a cow is administered hormones with $FSH$-like activity to induce superovulation.
Instead of one egg,which they normally yield per cycle,they produce $6-8$ eggs.
The animal is then mated with an elite bull or artificially inseminated.
The fertilized eggs at $8-32$ cell stages are removed non-surgically and transferred to surrogate mothers.
This allows the genetic mother to be available for another round of superovulation,thus improving the breed.

(C) The Southern blot technique is a method used to detect specific $DNA$ sequences in a sample.
The process begins with the isolation of $DNA$ from the sample,followed by the digestion of the $DNA$ using restriction enzymes.
Restriction enzymes act as molecular scissors that cut the $DNA$ at specific recognition sites,resulting in $DNA$ fragments of varying lengths.
After digestion,these fragments are separated based on their size using gel electrophoresis.

(D) $Bt$ cotton refers to the cultivation of genetically modified cotton plants.
The prefix $Bt$ stands for the bacterium $Bacillus$ $thuringiensis$.
This bacterium naturally produces a specific protein that acts as an endotoxin,which is toxic to certain insect pests such as lepidopterans,coleopterans,and dipterans.
By inserting the gene responsible for this endotoxin into the cotton plant,the plant becomes resistant to these pests,reducing the need for chemical insecticides.

(A) Animals often adopt various strategies to survive in hostile environments,such as camouflage,mimicry,or warning coloration.
Camouflage is a strategy where an organism blends with its surroundings to remain unnoticed by predators or prey.
The Praying mantis is green in colour,which allows it to merge with plant foliage (camouflage),thereby protecting it from predators and helping it hunt.
Since the Praying mantis's green colour is a specific example of a strategy used to survive in a hostile environment,the Reason correctly explains the Assertion.

(C) If the Bengal tiger becomes extinct,its unique genetic information,known as the gene pool,will be lost forever. Biodiversity conservation is essential because species represent a unique evolutionary history. Methods for conservation include in-situ conservation (e.g.,National Parks,Wildlife Sanctuaries) and ex-situ conservation (e.g.,zoological parks,gene banks,and cryopreservation).

(A) $DDT$ (dichloro diphenyl trichloroethane) is an organochlorine contact insecticide that acts as a nerve poison.
It was first synthesized in $1874$ and its insecticidal properties were discovered in $1939$.
$DDT$ was indeed used on a wide scale for the first time during World War $II$ to control diseases like typhus and malaria,and subsequently,it was used extensively in agriculture to protect crops from pests,which significantly increased agricultural output.
Therefore,both the Assertion and the Reason are correct,and the Reason provides a valid context for the widespread use and subsequent impact of $DDT$ on agricultural productivity.