单项选择题

Jet belts and rocket belts were developed （）

A．20-30 years ago
B．50 years ago
C．recently
D．30 years ago

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单项选择题
The Solo Flyer is able to lift off the ground by using （）

A．a solar powered engine
B．engine-powered twin fans
C．large flapping wings
D．rotating blades
单项选择题
Robert Fulton's invention, an airphibian, can best be described as （）

A．a helicopter
B．a solo flying machine
C．a flying car
D．an airplane
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