5. Air transport companies use different plans ____.
6. The United States was the country where ____.
7. The forty years from the 1930s to the 1970s was an important period ___.
8. Nowadays airports provide all kinds of services ____
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
参考答案： C E D A D B A C
