蒸汽管道和给水管道上必须有足够的（）
A.调节阀
B.空气阀
C.安全阀
D.疏水阀
参考答案：B, C, D
31. All books ______ to the library more than three days late will be subject to a fine.
A．return
B．returning
C．to return
D．Returned
— Didn't they come to the party last week?
— Yes. They didn't want to come with us at first, but then we ________ persuade them. [ ]
A. would
B. could
C. were able to
D. had to
