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

问题补充：

—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

答案：

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.