[单项选择]

Passage 5
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 in directly, 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 power. Coal represents 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 with the sun&rsq
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