In the human eye,............. absorbs $UV-B$ radiation.

$A$ - Micro-organisms involved in the biodegradation of organic matter in the receiving water body consume a lot of oxygen,and as a result,there is a sharp decline in dissolved oxygen in the stream from the point of sewage discharge.
$R$ - $BOD$ decreases when the dissolved oxygen amount decreases in water.

Gastro-intestinal disturbances are caused by:

The Bhopal gas tragedy occurred in $1984$ because methyl isocyanate reacted with:

$A$: Biomagnification of $DDT$ can enhance the decline in bird population.
$R$: $DDT$ causes thinning of eggshells and their premature breaking by disturbing calcium metabolism.

What is meant by the aging of a lake?