Which of the following is significant for water pollution?

Lead is considered as

What is the troposphere,and which components are present in it?

Identify the coldest region among the following layers of atmosphere.

What is the greenhouse effect?

Given below are two statements.
Statement $I:$ In polluted water,values of both dissolved oxygen and $BOD$ are very low.
Statement $II:$ Eutrophication results in a decrease in the amount of dissolved oxygen.
In the light of the above statements,choose the most appropriate answer from the options given below.