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

单选材料题

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：

本题解析：

【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。