问题
选择题
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
答案
答案：B
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。