Before the first non-stop flight made in 1949, it ______necessary for all planes

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问题 Before the first non-stop flight made in 1949, it ______necessary for all planes to land for refueling.

选项 A、would be
B、has been
C、had been
D、would have been

答案 C

解析由before引导的时间状语从句是一般过去时，所以主句应该采用过去完成时。

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