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 nations 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.

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