Tony full marks but he was so careless as to make a spelling mistake.A. could have gain

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。