液压冲击产生的原因有以下哪几项？
A. 突然换向
B. 阀门突然关闭
C. 溢流阀失灵
空穴现象常出现在什么地方
A. 阀口
B. 液压缸
C. 泵的入口
D. 节流阀
I did not like the house at the beginning but the new furniture ________ all the difference.
A. took
B. made
C. did
D. had
I stopped smoking for the ________ of health.
A. sake
B. reason
C. cause
D. purpose
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