Water filled in two glasses $A$ and $B$ have $BOD$ values of $10$ and $20$ respectively. The correct statement regarding them is:

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.

The chemical substance of photochemical smog responsible for eye irritation is

If the concentration of nitrate in drinking water exceeds $45 \ ppm$,which disease is likely to occur in children?

Identify the correct statements from the following:
$I$. Photochemical smog has high concentration of oxidising agents
$II$. $NO_2$ is present in classical smog
$III$. Higher concentration of $SO_2$ in air can cause stiffness of flower buds
$IV$. pH of rain water is approximately $7.5$

What is the correct sequence of atmospheric layers starting from the Earth's surface?