Which is the main causative factor of desertification?

$A$: $A$ few toxic substances often present in industrial waste waters undergo biological magnification.
$R$: $A$ toxic substance can easily be metabolised or excreted by an organism.

World Environment Day is celebrated on

$DDT$ residues are rapidly passed through the food chain causing biomagnification because $DDT$ is:

In India,the most commonly used pesticide $BHC$ is

Photochemical smog formed in congested metropolitan cities mainly consists of