钻头送入孔内（或定位孔内）开始钻进时，压力不宜过大，要轻压慢转，待钻头下到（）工作平稳后，压力再逐渐增大。
A.孔口
B.孔内
C.孔底
参考答案：C
Haven’t you enough common ________ to know that it is unwise to go outside in such a hazy (雾霾的) day? You may get lost.
A．practice
B．sense
C．rule
D．goal.
往复式压缩机吸排气阀有敲击声是什么原因？
