题目内容：
The reasonwhy all the solar energy falling on the earth can’t be utilized is that ______．根据以下内容，回答下面的题目。The energy which the sun radiates goes inevery 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 everymiunte as mankind utilizes in a year．At present, we usethis energy indirectly, and it is our final source of power．Coal represents the chemical action of the sun on green plantsthousands of years ago．Water power results from therain formed by vapor which comes from the evaporation of water under thesunshinE．Even windmills operate because of air currentsset in motion by the different heating affects of the sun in different places．Some day, through chemistry or some type of solar engine, we shallharness this great source of energy more directly．Alreadya scientist has worked out an engine, surprisingly efficient, in which the sun’srays are concentrated through mirrors on a tube of water to create steam．How muchenergy given off by the sun reaches the earth?A．only a verysmall part is available
B．the sun istoo distant from the earth
C．it takesthousands of years for this energy to be stored up in coal and oil
D．ourscientific knowledge is not sufficient
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