______ a new house is impossible for the young couple because they haven't saved enough money．

A．Buy

B．Buying

C．Bought

D．Having bought

根据经验，为了满足船舶具有适度的稳性，对具有三层舱的船舶来说，其底舱应占全部货量的（）。

A.30%

B.40%

C.55%

D.70%