The average atmospheric residence time is lowest for which of the given greenhouse gases?

Polar stratospheric clouds were observed over Antarctica some time ago. How might they have formed,and what happens if some of these clouds break down due to heat in the presence of sunlight?

Highly polluted water could have a $BOD$ value of: (in $ppm$)

Fill in the blanks:
$(1)$ Deficiency of fluoride is responsible for ............. .
$(2)$ The upper limit of lead concentration in drinking water is ............. .
$(3)$ The maximum limit of nitrate in drinking water is ............. .
$(4)$ ............. and ............. chemicals act as poisons for the nervous system.

What is $DDT$ among the following?

The type of pollution that increases during the daytime and in the presence of $O_{3}$ is