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正确答案C
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正确答案C
