在起下时，如果中止作业先在井口管子接上（），然后再进行其他作业。
A、闸门
B、安全阀
C、旋塞
参考答案：B, C
_______caused the accident is still a complete mystery.
A．What
B．That
C．How
D．Where
I've made a mistake, though _____ . [ ]
A. interided to
B. intended
C. didn't intend
D. didn't intend to
