单项选择题

The word "they" in line 11 refers to ______.

A．models
B．conditions
C．events
D．regions
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单项选择题
Why does the author state in line 9 that observations are taken just once every twelve hours
A．to show why the observations are of limited value.
B．to compare data from balloons and computers,
C．to give an example of international cooperation,
D．to indicate that the observations are timely.
单项选择题
What is the main topic of the passage
A．dangerous weather.
B．computer and weather.
C．weather forecasting.
D．weather-balloon observations.
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