What is $DDT$ among the following?

Salts of chromium and arsenic are generated as waste from which industry?

$CF_xCly$ [where $x + y = 4$] compounds are not useful because .....

$A$ large number of fish are suddenly found floating dead on a lake. There is no evidence of toxic dumping,but you find an abundance of phytoplankton. Suggest a reason for the fish kill.

The term biosphere is used for the zone of the earth where life exists:

Dissolved oxygen is essential for aquatic life. What are the reasons responsible for the depletion of dissolved oxygen in water?