Before the first non-stop flight made in 1949, it ______ necessary for all plane

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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

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[originaltext]M:Myflightleavesat4:30.W:Thenyoudbetterleavefortheair