$DDT$ residues are rapidly passed through food chains causing biomagnification. What is the reason for this?

In agriculture,improper drainage of water leads to:
$I-$ The soil becomes waterlogged.
$II -$ The soil surface draws up salts from the water.
$III -$ Salts are deposited as a thin crust on the land surface.
$IV -$ Salts start accumulating around the roots of the plants.
$V -$ The increasing concentration of salts becomes favorable for crop growth.
$VI -$ It is highly beneficial for agriculture.
Which of the above statements are correct?

If the concentration of lead in the blood reaches ........,it is considered a warning sign.

Which of the following is not used as a disinfectant in drinking water?

In a eutrophic water body,the excessive growth of $\underline {X}$ leads to the death of $\underline {Y}$.

$dB$ is a standard abbreviation used for the quantitative expression of