单项选择题

What does "clear-air turbulence" probably mean (Paragraph 1)

A．A not very rough storm.
B．Unexpected disturbed air.
C．A kind of visible storm.
D．A storm over mountain ridges.
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单项选择题
In December 1997, a United Airlines flight hit unexpected rough air,
A．causing a lot of damage to the plane.
B．throwing its passengers out of the cabin.
C．resulting in heavy casualties.
D．forcing the pilot to make an emergency landing.
单项选择题
The last paragraph tells us, among other things, that
A．the lidar detector needs improvement.
B．many airlines are interested in the system.
C．passengers often forget to fasten their seatbelts.
D．the lidar detector can be used in a wide range of areas.
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