The forty years from the 1930s to the 1970s was an important period
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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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正确答案：正确D
【答案】D【解析】从第三段可知，在19世纪30年代到70年代，出现了国际航班，远距离运输服务，以及新一代“喷气机”运输和超音速运输服务，由此可知，这一时期开始提供多种不同类型的运输服务。所以本题选D。