单项选择题

According to the speaker, what must balloon pilots be careful to do

A．Watch for changes in weather.
B．Watch their attitude.
C．Check for weak spots in the balloons.
D．Test the strength of the ropes.
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单项选择题
Schaefer discovered that ______.
A．dry ice serves well as centers for raindrops
B．dry ice is a cheap material to make rain
C．rain can be created without adding centers to the moisture
D．rain can not be made without adding centers to the moisture
填空题
The sun s energy.
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