问题：
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
● 参考解析
【答案】A
【解析】根据第一段可知，航空公司利用飞机运输旅客，货物和邮政。运营的飞机从单引擎到大型多引擎飞机。A为“在不同运输发展阶段使用不同飞机”符合逻辑和题意。因此，本题选A。
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