Question

—I knocked but nobody answered the door.
----They _____asleep then and didn’t hear you.

A．must be

B．should be

C．must have been

D．should have been

Answer 1

C

考察虚拟语气.句意:我敲门但是没有人开门._那时候他们一定是已经睡着了,没有听到你敲门. “must+have+done”表示对过去事情的肯定推测，译成“一定做过某事”故选C.