Substances that cause pollution or contaminate the environment are called .........

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

The $COD$ (Chemical Oxygen Demand) of a water sample is determined using:

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:

The photochemical smog does not generally contain.

Identify the types of smog that are oxidizing and reducing in nature.