问题补充：
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
答案：
A. would be
B. has been
(答案→)C. had been
D. would have been
解析：考查时态：句意：在1949年世界上首次不间断飞行之前, 所有飞机有必要的降落加油。说明“有必要”是在1949年之前，用过去完成时，选C。
