--- He worked out such different questions at a very young age.
--- Oh, he a very smart student then.
A. could have been
B. should have been
C. might have been
D. must have been
网友回答
A. could have been
B. should have been
C. might have been
(答案→)D. must have been
解析：考查情态动词表示推测。Could have done本可以做某事，实际上却未做；should have done本应该做某事，实际上却未做；AB两项表示的是虚拟语气。CD两项表示的是对过去情况的推测。might have done也许做了某事；must have done一定做了某事；句意：—在如此小的年龄，他算出了如此困难的问题。—那么他阅读是一个非常聪明的学生了。故D正确。