单项选择题

We learn from the third paragraph that ______.

A．London was plagued with rats in the 1950s
B．London is covered with smog today
C．London was polluted by smog in the 1950s
D．Los Angeles is as heavily polluted today as London was in the 1950s

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单项选择题

What will happen if the dust aerosol layer develops

A．The earth will get extremely hot.
B．The balance between the amount of heat absorbed from the Sun and the amount lost could hardly be maintained.
C．The light of the Sun could no longer reach the surface of the Earth.
D．Infrared radiation could no longer be transmitted from the Earth to outer space.

单项选择题

The author's main purpose in writing the article is to warn of ______.

A．warm weather
B．hot weather
C．a new ice age
D．a new iceberg