问题补充:
Before the first nonstop flight made in 1949,itnecessary for all planes to land for refueling.
A. would be
B. has been
C. had been
D. would have been
答案:
A. would be
B. has been
(答案→)C. had been
D. would have been
解析:本题时间状语从句为一般过去时,主句的情况是在从句之前,是“过去的过去”,因此用过去完成时。