—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
网友回答
A. must be
B. should be
(答案→)C. must have been
D. should have been
解析：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.