允许掘进距离是指从探水线开始向前方打探钻水一次打透积水的安全距离。
参考答案：错
There was going to ___ a struggle between her two friends. She doesn't know __ to deal with it. [ ]
A. be, how
B. has, what
C. be, what
D. have, how
单向阀的作用是（）。
A、减压
B、降温
C、保持系统压力
D、锁紧油路
