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
The problem of population is mainly man-made.
A．Y
B．N
C．NG
单项选择题
A．Replacing the native corn with the hybrid corn.B．Replacing the hybrid corn with the native corn.C．Replacing the new type of corn with the traditional corn.D．Replacing the traditional corn with the native corn.
A．Replacing the native corn with the hybrid corn.
B．Replacing the hybrid corn with the native corn.
C．Replacing the new type of corn with the traditional corn.
D．Replacing the traditional corn with the native corn.
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