离心泵停运时，应先（）泵出口阀门，后按停机电钮停机。
A、打开
B、关闭
C、关闭打开没有关系
D、无法确定
参考答案：B
If the green light doesn’t come on, you ______ go across the street.
A．must
B．needn’t
C．mustn’t
D．have to
. No one except two students ______ the meeting.
A．has been late for
B．have been late for
C．was late for
D．were later for
