问题补充：
I lived with my sister this summer and didn’t have to pay rent. So I save most of my salary.
A. could
B. would
C. was able to
D. should
答案：
A. could
B. would
(答案→)C. was able to
D. should
解析：本题考查can和be able to的差别。
① can和be able to都可以表示有能力作某事，但be able to的主语必须是人或动物，而can 没有这个限制，如：
He can swim. = He is able to swim.
The radio can be fixed in 30 minutes.（句中的can不能用be able to）
② 根据be动词的变化而有各种时态的变化，而can只有过去时。
③ be able to表示克服困难或通过努力做成某事，其过去时可表示“设法做成”，相当于managed to do或succeeded in doing，而can表示本身具有能力做某事。如：
With the help of the firemen, they were able to leave the burning house.
本题是指通过节省房钱来攒钱，当然用be able to更合适，再如：
Although his leg was hurt, he was able to swim to the bank.
