Ⅱ. 单项填空（共15小题; 每小题1分, 满分15分）
21. —He worked out such difficult 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
【解析】选D。考查情态动词。前一句句意为“他很小的时侯就能解决这类很难的问题”。后一句是对他以前的情况进行推测“噢, 他那时肯定是一个很聪明的学生”。所以应该用must have done/been的形式。should have done “本应该……”; could have done “本能够……”; might have done 虽然是对以前的情况进行推测, 但是语气很弱, 与句意不符。
