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。
