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:

Given below are two statements:
Statement $I$: An allotrope of oxygen is an important intermediate in the formation of reducing smog.
Statement $II$: Gases such as oxides of nitrogen and sulphur present in the troposphere contribute to the formation of photochemical smog.
In the light of the above statements,choose the correct answer from the options given below:

The living organisms on or around the earth constitute

Mention the types of pollution.

How do chlorofluorocarbons (CFCs) deplete the ozone layer in the stratosphere? Explain with the associated chemical reactions.

Which of the following is a lung irritant that can lead to an acute respiratory disease in children?