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
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