20.

　　A) The earth uses only a fraction of the sun's energy.

　　B) Early man was incapable of using solar energy.

　　C) Solar radiation can be dangerous.

　　D) The windmill was the forerunner of the steam engine.

　　21.

　　A) A large portion.

　　B) Five million horsepower.

　　C) As much as man uses in a year.

　　D) As much as man uses in a day.

　　22.

　　A) They use it indirectly.

　　B) They use it by a surprising engine.

　　C) By the chemical action of the sun.

　　D) They use it directly.

　　聽力原文：

　　Passage 5

　　One of the most popular myths about the United States in the 19th Century was that of the free and simple life of the farmer. It was said that the farmers worked hard on their own land to produce whatever their families' needed. They might sometimes trade with their neighbors, but in general they could get along just fine by relying on themselves, not on commercial ties with others. This is how Thomas Jefferson idealized the farmer at the beginning of the 19th century. And at that time, this may have been close to the truth especially on the frontier. But by the mid century sweeping changes in agriculture were well under way as farmers began to specialize in the raising of crops such as cotton or corn or wheat. By late in the century revolutionary advances in farm machinery has vastly increased production of specialized crops and extensive network of railroads had linked farmers throughout the country to markets in the east and even overseas. By raising and selling specialized crops, farmers could afford more and finer goods and achieve a much higher standard of living but at a price. Now farmers were no longer dependent just on the weather and their own effort, their lives were increasingly controlled by banks, which had powder to grant or deny loans for new machinery, and by the railroads which set the rates for shipping their crops to market. As businessmen, farmers now had to worry about national economic depressions and the influence of world supply and demand on, for example, the price of wheat in Kansas. And so by the end of the 19th century, the era of Jefferson's independent farmer had come to a close.