问题补充:
a new house is impossible for the young couple because they haven saved enough money.
A. Buy
B. Buying
C. Bought
D. Having bought
答案:
A. Buy
(答案→)B. Buying
C. Bought
D. Having bought
解析:句意:对年轻的夫妇来说买一个新房子是不可能的,因为他们没有攒够足够的钱。这里的is impossible是谓语,前面的内容是主语,用动名词做主语,所以选B。