-Tom graduated from college at a very young age .

题目

-Tom graduated from college at a very young age .
-Oh,he ——a very smart boy then .
A、must have been B、might be C、should have been D、could be

答案

A
肯定那是是个聪明的男孩.
B是可能是 但表示 一种状态 或者 现在的情况（be)
C.本应该should have been
D 可能是 /可以成为.