Photochemical smog consists of an excessive amount of $X$ in addition to aldehydes,ketones,$PAN$,etc. $X$ is:

Which among the following statements about ozone depletion is $NOT$ true?

Methyl mercaptans are generated as waste from which industry?

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R).$
Assertion $(A):$ Photochemical smog causes cracking of rubber.
Reason $(R):$ Presence of ozone,nitric oxide,acrolein,formaldehyde and peroxyacetyl nitrate in photochemical smog makes it oxidizing.
Choose the most appropriate answer from the options given below:

Ozone depletion in the stratosphere is mainly caused by

Water is considered to be polluted if the dissolved oxygen $(D.O.)$ content is less than $...$ $ppm$ :