Before the first non-stop flight made in 1949, it ________

来源：百度知道编辑：UC知道时间：2024/05/17 08:43:08

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

选C had been
在1949年制造出空中加油机之前，所有飞机必须降落补充燃油。
Before the first non-stop flight made in 1949是过去表达，所以在之前要用过去完成时

写错，sorry

C, 过去完成时

为啥是D

我怎么觉得是C

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