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．Air pollution laws in the United States and in Canada typically don't specify the amount of pollutants released by industries.
B．The neighboring areas of a factory receive less pollution than distant areas.
C．The number of occurrences of acid rain over the last few years has been on the rise
D．It is generally believed that ineffective air pollution laws rather than industries are to blame for acid rain.