下列液压阀除进出口外还必须有泄油口的是（）。
A．先导型溢流阀
B．先导型减压阀
C．调速阀
D．直动型卸荷阀
参考答案：B
The children have homework to do every day. [ ]
A. many too
B. too many
C. much too
D. too much
电缆与铁路、公路、街道、厂区道路交叉时．应敷设在坚固的（）内。
A.保护管
B.地沟
C.架空
D.隧道
E.阀门井
