At last he finish the work ahead of time.
A. could
B. was able to
C. could have done
D. would have been able to
网友回答
A. could
(答案→)B. was able to
C. could have done
D. would have been able to
解析：此句检测对can和be able to辨析使用的能力。从时态上讲，can只有过去，而be able to可用于各种时态；从语意上讲，be able to的过去可表示“设法完成某事”，相当于managed to do sth.而could则无此用法。