Agriculture is the number one industry in the United States and agricultural products are the country's leading export. American farmers manage to feed not only the total population of the United States, but also millions of other people throughout the rest of the world. Corn and soybean exports alone account for approximately 75 percent of the amount sold in world markets.

　　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.

　　Each field is covered by a limited amount of topsoil, the upper layer of earth which is richest in the nutrients 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 decreasing the supply of topsoil. In the 1830s, nearly two feet of rich, black top soil 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 percent 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 of 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 drainage from eroding fields. Furthermore, fertilizers only replenish the soils they do not prevent its loss.

　　26. The last sentence in the first paragraph gives an example to show_______.

　　A. that American farmers manage to feed the total population of the U. S.

　　B. the leading position of the U. S. farming in the world

　　C. how important American people consider their farming

　　D. that many people in the world rely on the export of the agricultural products of the U. S. A.

　　27. In order to compensate for the loss of fertile topsoil, farmers have been_______.

　　A. planting less corn and soybean B. putting fertilizers on their fields

　　C. preventing soil erosion D. decreasing the supply of top soil

　　28. At the present rate, approximately how many years later the black top soil now covering the Midwest will completely be blown or washed away?

　　A. 120 years later. . B. 80 years later.

　　C. 50 years later. D. 100 years later.

　　29. "This practice" in Paragraph 4 refers to_______.

　　A. that farmers have lowered the yield of corn and soybean

　　B. that farmers have expanded croplands

　　C. that farmers have applied more chemical fertilizers

　　D. that the top soil has been decreased greatly

　　30. All of the following are statements about the disadvantages of fertilizers EXCEPT that

　　A. they replenish the soil B. they do not prevent the loss of soil

　　C. they are destroying the ecology D. they pollute the nation's water supply

　　答案：　26. D 27. B 28. B 29. C 30. A