在液体流动中，因某点处的压力低于空气分离压而产生大量气泡的现象，称为（）。
A、层流
B、液压冲击
C、空穴现象
D、紊流
参考答案：C
— When shall we go to watch the basketball match in the sports centre?
— Not until the work _____ tomorrow. [ ]
A. will be finished
B. is finished
C. will finish
D. has finished
研磨后的密封带宽度：进气阀应在1.2~2mm之内，排气阀应在（）之内，密封带过宽则密封性不好，密封带过窄则工作寿命短。
A.0.5~1.2mm
B.1.2~2mm
C.2~3mm
D.2.3~3.2mm
