—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

存在工人和资本家时，马克思的两个部类模型一般是指简单再生产和（）。

A.扩大再生产

B.强化再生产

C.机械再生产

D.自动再生产