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 absent in polluted water?

$DDT$ is a

Biochemical Oxygen Demand $(BOD)$ is a measure of:

The 'Blue Baby' syndrome is caused by .........