Question

--- 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

Answer

D

【解析】

试题分析：句意：--在这么小的年龄他就能提出不同的问题。--哦，那他当时一定是非常聪明的。could have done过去本可以做某事而未做成； should have done 指过去应该做某事而实际未做；might have done 表示过去可能做某事；must have done表示过去一定能做某事。根据then可知这是指at a very young age，所以表示动作已经过去了，根据第一句可知这里推测的可能性很大，故选D。

考点：考查情态动词的用法。