The boy the first prize, but he was so careless as to make a foolish mistake.
A. must have gained
B. need have gained
C. could have gained
D. shall have gained
网友回答
A. must have gained
B. need have gained
(答案→)C. could have gained
D. shall have gained
解析：考查情态动词。must have done 对过去事情肯定的推测，一定，need have done 本需要做某事，而实际上没有做，could have done 本可以做某事，而实际上没有做，句意：这个男孩本可以获得第一名的，但是实际上没有做。故选C。