Salinity of the soil is often caused due to

Explain biomagnification and its effect in $DDT$.

If the concentration of $DDT$ in water is $0.003\, ppm$,what could be its concentration in small fish due to biomagnification (in $, ppm$)?

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?

Which of the following is a secondary air pollutant?

What is the major drawback of $DDT$ as an insecticide?