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

A、inthedevelopmentofairtransportation

B、theearliestpassengerflightsweresuccessfullyoperated

C、tomaketraveleasyandpleasantforthepassengers

D、toprovidedifferentservices

E、theshortageofqualifiedpilots

F、travelingbyairwasverycheap