液压泵产生困油现象的原因是什么？
参考答案：
存在闭死容积，且闭死容积的大小发生变化。
如果用电设备的额定电压与电网的额定电压不符，或电网电压偏移过大，将使设备不能正常工作，甚至发生设备或人身事故。
It has been announced that candidates _______ remain in their seats until all the papers have been collected. [ ]
A. can
B. will
C. may
D. shall
