He it on schedule, but somehow he fell behind.
A. must have done
B. might finish
C. could have finished
D. could finish
网友回答
A. must have done
B. might finish
(答案→)C. could have finished
D. could finish
解析：句意为：他本能够按时完成任务，但他莫名其妙的落后了。must have done 对过去发生的事情的肯定推测，译为“一定---”;might do对现在发生的事情的肯定推测，译为“也许---”语气不十分肯定；could have done意为“本能够---”；could do 用于虚拟语气，对现在或将来的事情虚拟。