Which of the following statements are correct?
a) $F^{-}$ ion concentration above $2 \ ppm$ causes brown mottling of teeth
b) The maximum limit of nitrate in drinking water is $400 \ ppm$
c) Depletion of ozone layer leads to cataract and skin cancer
d) The irritant red haze in the traffic is due to oxides of sulphur

The chemical substance of photochemical smog responsible for eye irritation is

Among the following, the one which is not a "green house gas" is:

Which aerosols deplete ozone?

Which of the following statements about photochemical smog is wrong?

Fill in the blanks:
$(1)$ $A$ special cloud is formed over the Antarctica region in winter,which is called ..............
$(2)$ $A$ known source of pollution is called ..............
$(3)$ An unknown source of pollution is called ..............
$(4)$ The concentration of dissolved oxygen in water is ......... and the concentration of dissolved oxygen in air is ............