Which is the best title for the passage

A．It is acknowledged by scientists to be the most serious pollution problem that human beings now face.
B．Many endangered species are on the verge of extinction because of acid rain.
C．Acid rain is composed of sulfur dioxide and nitrogen oxides mixed with water vapor found in the upper atmosphere.
D．Acid rain is mostly found in the United States and Canada.

A．Fish in the lakes die because acid rain deprives them of oxygen.
B．Fish in the lakes die from taking in poisonous substances released from the acids.
C．Fish in the lakes die from direct contact with accumulated acid in the water.
D．Air pollutants are dissolved in the water and thus kill the fish.