--- 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：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。