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

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

出自：三门峡职业技术学院英语读写

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