问题
选择题
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
答案：D
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。