离心泵采用后弯叶瓣，可使泵获得较高的（）。
A、效率
B、总压头
C、流量
D、动压头
参考答案：A
The man stopped from time to time_______whether he was being followed.
A．as if found
B．as to find
C．as if to find
D．when found
突然停电时，现场所有的程控阀将自动全部打开。
