Before the first nonstop flight made in 1949,it necessary for all planes to land for refueling.
A. would be
B. has been
C. had been
D. would have been
网友回答
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。