What is the volume percentage of nitrogen gas in the atmosphere (in $\%$)?

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R).$
Assertion $(A):$ Photochemical smog causes cracking of rubber.
Reason $(R):$ Presence of ozone,nitric oxide,acrolein,formaldehyde and peroxyacetyl nitrate in photochemical smog makes it oxidizing.
Choose the most appropriate answer from the options given below:

Which aerosols deplete ozone?

Which of the following is used as a green solvent?

Mention the type of pollutants of tropospheric pollution.

Biochemical Oxygen Demand,$(BOD)$ is a measure of organic material present in water. $BOD$ value less than $5 \ ppm$ indicates a water sample to be ..... .