Cars play an important role in modern life, but they also cause many problems like air pollution.
A. should
B. can
C. must
D. will
网友回答
A. should
(答案→)B. can
C. must
D. will
解析：考查情态动词的用法。should应该，can有时候可能会，must必须，will将会，句意：汽车在相待生活中起着非常重要的作用，但是，他们有时候可能也会引起许多为题像空气污染。故选B。