某国未清偿外债余额为1200亿美元，国民生产总值为8000亿美元，货物服务出口总额为1600亿美元，当年外债还本付息总额为400亿美元，则该国外债偿债率为（）。

A.15%

B.75%

C.25%

D.5%

- Tom, where is your father?

- I’m not sure. He _______ in his office.

A．must be

B．may be　

C．maybe

D．need be