Methyl isocyanate gas is used for synthesizing

Which one of the following is the heavy toxic metal present in waste water from industries?

$A$ lake with an inflow of domestic sewage rich in organic waste may result in:

$A$ : $PAN$ is a secondary pollutant.
$R$ : $PAN$ is formed by interaction of nitrogen oxides and hydrocarbons in the absence of light.

The diagram below shows a scrubber. Identify $A, B, C$ and $D$.

The main source of $BOD$ in the Ganga river is .....