What is the cause of the greenhouse effect?

The reagent used in the determination of the chemical oxygen demand $(COD)$ of a water sample is

Which of the following protects life on earth from harmful effects of $UV$ radiations from the sun?

Given below are two statements :
Statement $I :$ The $pH$ of rain water is normally $\sim 5.6$.
Statement $II :$ If the $pH$ of rain water drops below $5.6,$ it is called acid rain.
In the light of the above statements,choose the correct answer from the options given below:

The main pollutant from automobile exhaust is

$A$ farmer is using pesticides in his field. He uses the produce from his field as food for fish. He is told that the fish are not fit for human consumption because large amounts of pesticides were found in the fish cells. How is this possible? Explain.