This productivity, however, has its price. Intensive cultivation exposes the earth to the damaging forces of nature. Every year wind and water remove tons of rich soil from the nation's croplands, with the result that soil erosion has become a national problem concerning everyone from the farmer to the consumer.

　　Each field is covered by a limited amount of topsoil, the upper layer of earth which is the richest in the nutrient and minerals necessary for growing crops. Ever since the first farmers arrived in the Midwest almost 200 years ago, cultivation and, consequently, erosion have been depleting the supply of topsoil. In the 1830s, nearly two feet of rich, black topsoil covered the Midwest. Today the average depth is only eight inches, and every decade another inch is blown or washed away. This erosion is steadily decreasing the productivity of valuable cropland. A United States Agricultural Department survey states that if erosion continues at its present rate, corn and soybean yields in the Midwest may drop as much as 30% over the next 50 years.

　　So far, farmers have been able to compensate for the loss of fertile topsoil by applying more chemical fertilizers to their fields; however, while this practice has increased crop yields, it has been devastating for ecology. Agriculture has become one of the biggest polluters on the nation's precious water supply. Rivers, lakes, and underground reserves of water are being filled in and poisoned by soil and chemicals carried by the drainage from eroding fields. Furthermore, fertilizers only replenish the soil; they do not prevent its loss.

　　Clearly something else has to be done in order to avoid an eventual ecological disaster. Conservationists insist that the solution to this problem lies in new and better farming techniques. Concerned farmers are building terraces on hilly fields, rotating their crops, and using new plowing methods to cut soil loss significantly. Substantial progress has been made, but soil erosion is far from being under control.

　　Questions 21-25 are based on Passage Five.

　　21. Which of the following is NOT true?

　　A. Farmers are the only people who are concerned about soil erosion.

　　B. Farmers in the Midwest lead a very busy life and work long hours.

　　C. Farmers should use advanced techniques to produce more food.

　　D. Farmers in the U.S provide food for the U.S. and other countries.

　　22. Agricultural products of the U.S. ______.

　　A. consist mainly of corn, wheat and soybeans

　　B. enjoy the world reputation for the good quality

　　C. are the largest source of exports of the country

　　D. account for about 75% of those sold in world market

　　23. What is the downside of using chemical fertilizers according to the passage?

　　A. It worsens soil erosion.  B. It endangers people's health.

　　C. It poisons the crops  D. It pollutes water supply.

　　24. Which is true about topsoil in Midwest?

　　A. It is the richest in the U.S.  B. It is lacking nutrients.