The forty years from the 1930s to the 1970s was an important period

The forty years from the 1930s to the 1970s was an important period 查看材料

A.in the development of air transportation

B.the earliest passenger flights were successfully operated

C.to make travel easy and pleasant for the passengers

D.to provide different services

E.the shortage of qualified pilots

F.traveling by air was very cheap

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