问题
选择题
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
答案：D
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。