[单选,材料题] The reason why 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 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 miunte 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．Water power 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 affects 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．
How much energy given off by the sun reaches the earth?