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: