Which of the following is not an air pollutant?

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:

The insecticide $DDT$ is considered as . . . . . .

Which of the following is an air pollutant?

Thermal power plants can lead to:

Pollution can be controlled by