离心式水泵启动前，入口门应（）、出口门关闭，轴承室内油位在（）间。
参考答案：开启、1/2~2/3
There _____ many students on the playground at the moment. [ ]
A. is
B. are
C. was
D. were
气井的井口装置由（）、油管头和采气树组成。
