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。