单项选择题

A．Crowded air traffic.
B．The large size of airplanes.
C．Mistakes by air traffic controllers.
D．Bad weather.
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单项选择题
A．4,000 years ago.B．3,000 years ago.C．2,000 years ago.D．1,000 years ago.
A．4,000 years ago.
B．3,000 years ago.
C．2,000 years ago.
D．1,000 years ago.
填空题
B
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