Photochemical smog consists of an excessive amount of $X$,in addition to aldehydes,ketones,peroxyacetyl nitrate $(PAN)$,and so forth. $X$ is

Water pollution is caused due to

Which of the following molecules is present in photochemical smog?

Given below are two statements :
Statement $I :$ The $pH$ of rain water is normally $\sim 5.6$.
Statement $II :$ If the $pH$ 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:

Biochemical oxygen demand $(BOD)$ is a measure of organic materials present in water. $A$ $BOD$ value less than $5 \ ppm$ indicates a water sample to be:

Methane in polluted air reacts with ozone to form which of the following compounds?