The primary causes of environmental pollution include:

Give scientific reasons: The use of $CNG$ is more advantageous than the use of petrol.

$A$ - 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.

The below chart shows the sources of water pollution. Read the chart carefully and identify $A, B, C$ and $D$.

What is the full form of $BOD$?

$BHC$ and $DDT$ belong to which class of pesticides?