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。这里的肯定句表示“有时会，可能会”，用can。注意can与must都能用于肯定句中表示肯定的推测与判断，表示具体情形的推测判断用must，而can表示一般性的叙述，“有时会”。