How many ozone molecules are destroyed by a single chlorine free radical?

Which of the following protects life on earth from harmful effects of $UV$ radiations from the sun?

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 smog? How is classical smog different from photochemical smog?

Identify the correct statements from the following:
$a$) $BOD$ value of clean water is less than $5 \ ppm$
$b$) Oxidation of ethene in the presence of $Pd^{2+}$ catalyst in aqueous medium gives acetic acid
$c$) Photochemical smog causes damage to plant life
$d$) Reducing smog is a mixture of smoke,fog and $SO_2$

Smog is primarily caused by the presence of which of the following?