The light is still on. They to turn it off when they left the room.A. must forgetB. could

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。