The possibility of photochemical smog formation is more at

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 ............

Why is it necessary to measure the $BOD$ value of water?

Which one of the following statements is not correct?

Match the following.

Column $I$	Column $II$
$A$. $SO_2$	$I$. Photochemical smog
$B$. $PAN$	$II$. Acid rain
$C$. Smoke	$III$. Stratospheric pollutant
$D$. $CF_2Cl_2$	$IV$. Particulate

What is the volume percentage of nitrogen gas in the atmosphere (in $\%$)?