Before the first non-stop flig

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Before the first non-stop flight made in 1949,it ( ) necessary for all plane to land for refueling.
A.would be
B.has been
C.had been
D.would have been
这道题在考什么？

解题思路：表示过去某一时间或动作以前已经发生或完成了的动作。它表示动作发生的时间是“过去的过去”。
解题过程：
选C。句中从句谓语动词所表示的动作发生在1949年，主句谓语动词所表示的情况发生在此之前，所以用过去完成时。

最终答案：如上

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