问题补充:
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
答案:
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。