We finish it ahead of time, though it was such a tough task.
A. could
B. would
C. were able to
D. ought to
网友回答
A. could
B. would
(答案→)C. were able to
D. ought to
解析：考查情态动词。could 能够，would 会，were able to 成功做成什么事，ought to 应该，句意：我们成功的提前完成了任务，虽然这是一个如此艰难的任务。所以选C。