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  3. The energy which the sun radiates goes in every direction , and only a minute part of it falls on the earth. Even so, it represe

The energy which the sun radiates goes in every direction , and only a minute part of it falls on the earth. Even so, it represe

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问题     The energy which the sun radiates goes in every direction , and only a minute part of it falls on the earth. Even so, it represents power of about 5 , 000, 000 horsepower(马力)per square mile per day: the sun gives us as much energy every minute as mankind utilizes in a year. At present, we use this energy indirectly, and it is our final source of power. Coal represents the chemical action of the sun on green plants thousands of years ago. Waterpower results from the rain formed by vapor which comes from the evaporation of water under the sunshine. Even windmills operate because of air currents set in motion by the different heating effects of the sun in different places. Some day, through chemistry or some type of solar engine, we shall harness this great source of energy more directly. Already a scientist has worked out an engine, surprisingly efficient, in which the sun’s rays are concentrated through mirrors on a tube of water to create steam.
The reason why all the solar energy falling on the earth can’t be utilized is that______.
选项 A、only a very small part is available
B、the sun is too distant from the earth
C、it takes thousands of years for this energy to be stored up in coal and oil
D、our scientific knowledge is not sufficient
答案D
解析 落到地球上的太阳能不能全部加以利用的原因是……。依据文章最后一句可知，对太阳能的直接利用的科研才刚刚起步，因而不能全部加以利用的原因必定是科学技术还不足以做到，故选克们的科技知识还不够。其它选项：A.只有很少一部分可资利用(文章第二句第三句明确指出：“太阳每分钟给我们提供的能量数量等于人类一年使用的能量数量……”，这就告诉我们选项A与事实不符。)；B．太阳离我们太远(这和落到地球上的太阳篮不能全部加以利用已经无关了。)；C．要用数千年这些能量才能储存到煤炭和石油中去(这里提问说的是太阳能而不是经过动植物转化储存起来的太阳能)。
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