31. —How long_________each other before they_________married?

—For about a year.

A．have they known, get

B．did they know, were going to get

C．do they know, are going to get

D．had they known, got

()是最早系统地研究重大危险源控制技术的国家。

A.美国

B.中国

C.韩国

D.英国