Which is the greatest source of air pollution these days?

The primary pollutant that leads to photochemical smog is

Formation of photochemical smog involves the following reactions in which $A$,$B$,and $C$ are respectively:
$(i)$ $NO_2 \stackrel{h\nu}{\longrightarrow} A + B$
$(ii)$ $B + O_2 \rightarrow C$
$(iii)$ $A + C \rightarrow NO_2 + O_2$
Choose the correct answer from the options given below:

Which of the following is not a greenhouse gas $(GHG)$?

Explain in short: Pollution of the troposphere caused by hydrocarbons.

The normal rain water is slightly acidic and its $pH$ value is $5.6$ because of which one of the following?