使用胎头吸引术助产不应超过（）
A.1次
B.2次
C.3次
D.4次
E.5次
参考答案：B
There was so much noise in the room that the speaker couldn't make himself _____. [ ]
A. being heard
B. hearing
C. heard
D. hear
在DN80，工作压力为0.6MPа的氧气管道上应选用（）
A.闸阀
B.截止阀
C.隔膜阀
