A．Energy conservation.
B．Transportation of the future.
C．Strip cities.
D．Advantages of air transportation over railroads.

A．On short trips.
B．On long trips.
C．When flying over cities.
D．When flying at high altitudes,

A．It uses nuclear energy.
B．It rests on a cushion of pressurized air.
C．It flies over magnetically activated tracks.
D．It uses a device similar to a jet engine.