问题补充:
Tony full marks, but he was so careless as to make a spelling mistake.
A. could have gained
B. can have gained
C. must have gained
D. must gain
答案:
(答案→)A. could have gained
B. can have gained
C. must have gained
D. must gain
解析:句意:Tony本来可以得满分的,但是他太粗心了,犯了一个拼写错误。could have gained表示过去本来可以做的事而没做;can have gained表示对过去情况的怀疑或不确定;must have gained表示对过去发生的事情或状态进行推测,选A。