. He full marks but he was so careless as to make a spelling mistake.A. must have gained

问题补充：

. He full marks, but he was so careless as to make a spelling mistake.

A. must have gained

B. can have gained

C. could have gained

D. must gain

答案：

A. must have gained

B. can have gained

(答案→)C. could have gained

D. must gain

解析：考查情态动词及虚拟语气。could have done本可能做某事而实际上没有做，句意：他本可以得满分的，但是他是如此的粗心以至于犯了一个拼写错误。所以选C。