The author mentions people dying of air pollution in ______.

A．all of the effects of pollution on the human body
B．how pollution can be controlled successfully
C．when the atmosphere first became polluted
D．how to wash the black snow clean

A．civilization may be doomed if pollution is not controlled
B．pollution is more serious in Europe than it is in America
C．most people do not know that pollution is a serious problem
D．black snow is not so serious compared with white snow