Biochemical oxygen demand $(BOD)$ is a measure of organic materials present in water. $A$ $BOD$ value less than $5 \ ppm$ indicates a water sample to be:

$A$ water sample is considered the cleanest based on which of the following $BOD$ values (in $ppm$)?

Which of the following statements about smog is/are correct?
$(i)$ Smog formed on sunny days contains oxidizing molecules.
$(ii)$ Photochemical smog contains many reactive molecules.
$(iii)$ The main polluting components of smog are oxides of carbon.
$(iv)$ The presence of carbon monoxide in air leads to the formation of ozone in smog.

Biochemical Oxygen Demand $(BOD)$ is the amount of oxygen required (in $ppm$):

Given below are two statements :
Statement $I$ : If $BOD$ is $4\,ppm$ and dissolved oxygen is $8\,ppm$,then it is a good quality water.
Statement $II$ : If the concentration of zinc and nitrate salts are $5\,ppm$ each,then it can be a good quality water.
In the light of the above statements,choose the most appropriate answer from the options given below :

How do chlorofluorocarbons (CFCs) deplete the ozone layer in the stratosphere? Explain with the associated chemical reactions.