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21.

Which of the following is usually not an air pollutant?

A.	Hydrocarbons	B.	Nitrous oxide
C.	Sulphur oxide	D.	Carbon dioxide

22.

The maximum biological magnification of DDT through food web is seen in:

A.	Algae	B.	Bacteria
C.	Man	D.	Higher plants

23.

Organisms at the base of the food chain are:

A.	Photosynthetic plants	B.	Decomposers
C.	Carnivores	D.	Herbivores