问题
选择题
. 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
答案：C
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。