Their answers are exactly the same. One of them from the other.
A. must copy
B. must have copied
C. should copy
D. should have copied
网友回答
A. must copy
(答案→)B. must have copied
C. should copy
D. should have copied
解析：句意：他们的答案一模一样，一个人一定是抄袭另外一个人的。根据句意是对过去的情况的肯定推测，首先排除AC，因为它们是表示现在情况，must have done过去一定做了，should have done过去本应该做而没做，选B。