What are the major causes of water pollution? Explain.

Which components are present in stratosphere?

Given below are two statements :

Statement $I$ : If $BOD$ is $4\,ppm$ and dissolved oxygen is $8\,ppm$, then it is a good quality water.

Statement $II$ : If the concentration of zinc and nitrate salts are $5 ppm$ each, then it can be a good quality water.

In the light of the above statements, choose the most appropriate answer from the options given below :

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.

Write down the reactions involved during the formation of photochemical smog.