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  3. All the useful energy at the surface of the earth comes from the activity of the sun. The sun heats and feeds mankind. Each year

All the useful energy at the surface of the earth comes from the activity of the sun. The sun heats and feeds mankind. Each year

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问题     All the useful energy at the surface of the earth comes from the activity of the sun. The sun heats and feeds mankind. Each year it provides men with two hundred million tons of grain and nearly ten million tons of wood.
    Coal, oil, natural gas, and all other fuels are stored-up energy from the sun. Some was collect ed by this season’s plants as carbon compounds. Some was stored by plants and trees ages ago.
    Even waterpower derives from the sun. Water turned into vapor by the sun falls as rain. It courses down the mountains and is converted to electric power.
    Light transmits only the energy that comes from the sun’ s outer layers, and much of this energy that is directed toward the earth never arrives. About nine-tenths of it is absorbed by the atmosphere of the earth. In fact; the earth itself gets only one half-billionth of the sun’ s entire output of radiant energy.
For mankind, the sun is the source of ______.
选项 A、solar activity
B、all heat
C、all food
D、both B and C
答案D
解析 文章第一段第二句指出，太阳给人类以热量和食物。
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