The ozone layer exists up to $20 \ km$ above the Earth. Which of the following statements is true regarding the ozone layer and ozone?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R:$
Assertion $A:$ Polluted water may have a value of $BOD$ of the order of $17 \, ppm$.
Reason $R:$ $BOD$ is a measure of oxygen required to oxidise both the biodegradable and non-biodegradable organic material in water.
In the light of the above statements,choose the most appropriate answer from the options given below.

According to current information,what is the primary cause of atmospheric pollution in industrial cities leading to acid rain?

The greenhouse gas(es) is/are:
$A$. Carbon dioxide
$B$. Oxygen
$C$. Water vapour
$D$. Methane
Choose the most appropriate answer from the options given below:

Assertion : Photochemical smog is produced by nitrogen oxides.
Reason : Vehicular pollution is a major source of nitrogen oxides.

Which of the following statements is not true about classical smog?