题目内容：
Before the first non-stop 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
参考答案：
选择题：Before the first non-stop flight made in 1949, it
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A.would be
B.has been
C.had been
D.would have been