Cars play an important role in modern life but theyalso cause many problems like air poll

问题补充：

Cars play an important role in modern life, but theyalso cause many problems like air pollution．

A. should

B. can

C. must

D. Will

答案：

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”。