The concentration of dissolved oxygen in water for the growth of fish should be more than $X \ ppm$ and the Biochemical Oxygen Demand $(BOD)$ in clean water should be less than $Y \ ppm$. $X$ and $Y$ in $ppm$ are,respectively:

Identify the correct statements from the following:
$i$. In the presence of $UV$ light,$CF_2Cl_2(g)$ gives chlorine free radicals which will react with $O_3(g)$ to form $O_2(g)$.
$ii$. Drinking water with $10 \ ppm$ fluoride is better than drinking water with $1 \ ppm$ fluoride.
$iii$. The maximum permissible concentration of lead in drinking water is $50 \ ppb$.

Given below are two statements:
Statement $I:$ Classical smog occurs in cool humid climate. It is a reducing mixture of smoke,fog and sulphur dioxide.
Statement $II:$ Photochemical smog has components,ozone,nitric oxide,acrolein,formaldehyde,$PAN$ etc.
In the light of above statements,choose the most appropriate answer from the options given below:

Leakage of which gas was responsible for the Bhopal tragedy in $1984$?

Which of the following statements is $NOT$ true about acid rain?

What is smog? How is classical smog different from photochemical smog?