问题补充:
Before you leave the lab, the doors and windows shut in case of rain.
A. is
B. are
C. is to be
D. are to be
答案:
A. is
B. are
C. is to be
(答案→)D. are to be
解析:考查be to do结构。主语是the doors and windows,谓语要用复数,排除A,C;be to do sth.一定要做某事。句意:你离开实验室前,一定要把门窗关好,以免下雨。所以选D。
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