The energy which the sun radiates goes in every direction, and only a minute part of it falls on the earth. It is reported that some scientists believe less than 1% of the solar energy fallen on the earth has been used so far. Even so, it represents power of approximately 5, 000,000 horsepower per square mile per day; the sun gives us as much energy every minute as mankind utilizes in a year. At present, because of the lack of scientific knowledge, we use this energy indirectly, and it is our only final source of power. Coal represents the chemical action of the sun on green plants thousands of years ago. Waterpower results from the sun’s creating vapor and subsequent rain. Even windmills operate because of air currents set in motion by the different heating effects of the use this immense source of energy more directly. Already, scientists have worked out a new type of energy, surprisingly efficient, in which the sun’s rays are concentrated through mirrors on a tube of water to create steam. Man cannot directly utilize the solar energy falling on the earth because ______.

A. only a small part of the solar energy reaches the earth
B. the sun produces different heating effects in different places
C. the sun is too distant from the earth
D. scientific knowledge is insufficient