填空题 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

填空题 Air transport companies use different plans______

填空题 The United States was the country where______

填空题 The forty years from the 1930s to the 1970s was an important periods______

填空题 Nowadays airports provide all kinds of services______