问题补充:
The light is still on. They to turn it off when they left the room.
A. must forget
B. could have forgotten
C. may forget
D. must have forgotten
答案:
A. must forget
B. could have forgotten
C. may forget
(答案→)D. must have forgotten
解析:考查虚拟语气。结合上文,灯仍然亮着,可知是对过去的事情的推测。“must+have+done”表示对过去事情的肯定推测;“could+have+done”表示对过去事情的假设。结合题意可知下文是推测语气,故选D。