Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason $\left(\mathrm{R}\right)$
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 :

Given below are two statements :
Statement $\mathrm{I}$ : Chlorofluoro carbons breakdown by radiation in the visible energy region and release chlorine gas in the atmosphere which then reacts with stratospheric ozone.

Statement $\mathrm{II}$ : Atmospheric ozone reacts with nitric oxide to give nitrogen and oxygen gases, which add to the atmosphere.

For the above statements choose the correct answer from the options given below :

Given below are two statements:

Statement I: The $\mathrm{pH}$ of rain water is normally $~5.6$.

Statement II : If the $\mathrm{pH}$ of rain water drops below $5.6$ , it is called acid rain.

In the light of the above statements, choose the correct answer from the options given below:

Given below are two statements:

Statement I : An allotrope of oxygen is an important intermediate in the formation of reducing smog.

Statement II : Gases such as oxides of nitrogen and sulphur present in troposphere contribute to the formation of photochemical smog. In the light of the above statements, choose the correct answer from the options given below:

Given below are two statements:
Statement I: The value of the parameter "Biochemical Oxygen Demand (BOD)" is important for survival of aquatic life.

Statement II: The optimum value of BOD is $6.5 \mathrm{ppm}.$

In the light of the above statements, choose the most appropriate answer from the options given below: