The forty years from the 1930s to the 1970s was an important period根据材料，完成下面的题目。Air Transportation1. Airplanes are used to carry passengers, cargo and mail. Air transport companies operate scheduled airlines and non-scheduled services over local, regional, national, and international routes. The aircraft operated by these companies range from small single-engine planes to large multiengine jet transports.2. The first air passenger services began in 1910, when dirigibles began operation between several German cities. The first scheduled airplane service to carry passengers began in the U. S in 1914. Several experimental airmail flights took place in India, Europe, and the United States before World War I, but air transport service did not become a true business until after the war.3. During World War Two, intercontinental air transport became firmly established. After the war the new long-distance transports with advanced facilities were increasingly able to avoid storms and strong wind and make flights more economical and consistent. A new generation of "jumbo-jet" transports began operations in 1970, and the supersonic transport entered passenger service in 1976.4. During the 1970s the number of domestic passengers on U. S airlines increased about 78%, and during the 1980s the figure was up about 58%. In 1990 there were 41.8 million international passengers, the figure was a 75% increase over 1980. The total cargo flown by U. S airlines almost doubled during the 1980s, from 5.7 billion to 10.6 billion ton-miles in 1990.5. Major airports provide a wide range of facilities for the convenience of millions of travelers. These range from such basic services as ticket-sales counters and restaurants to luxury hotels, shopping centers and play areas for children. International airports must also have customs areas and currency exchange counters and so on.Paragraph 2__________.

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