Which of the following is not a common component of photochemical smog?

The delicate balance of $CO_2$ and $O_2$ is $NOT$ disturbed by:

The chlorine free radical reacts with ozone to form a ..... free radical.

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 $SO_2$

Identify the incorrect statement.

State the measures to control environmental pollution.