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
An analogy was used to compare the way one could fly in the Solo Flyer to ______.
A．how a helicopter flies
B．how the Jetsons fly
C．how the airphibian flies
D．how Superman flies
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
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