单项选择题
How much energy given off by the sun reaches the earth
A．100 % but indirectly
B．5 %
C．50 %
D．only a very tiny amount
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单项选择题
In order to support this argument, the writer gives the example that ______ .
A．a machine using solar energy efficiently has already been developed
B．the sun is our final source of power
C．man uses only a tiny part of the sun's energy
D．the part of the sun's energy we use is used indirectly
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单项选择题
The main idea of the whole story is that ______ .
A．there is more food and water in space than on earth
B．eating and drinking in space is a special problem
C．astronauts can never eat or drink in a spaceship
D．there is no food or water in a spaceship
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