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【Paragraph 10】The extreme seriousness of desertification results from the vast areas of land and the tremendous　numbers of people affected, as well as from the great difficulty of reversing or even slowing the process. Once the soil　has been removed by erosion, only the passage of centuries or millennia will enable new soil to form. In areas whereconsiderable soil still remains, though, a rigorously enforced program of land protection and cover-crop planting may　make it possible to reverse the present deterioration of the surface. (Desert Formation)

　　Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

　　○Desertification is a significant problem because it is so hard to reverse and affects large areas of land and great　numbers of people.

　　○Slowing down the process of desertification is difficult because of population growth that has spread over large　areas of land.

　　○The spread of deserts is considered a very serious problem that can be solved only if large numbers of people　in various countries are involved in the effort.

　　○Desertification is extremely hard to reverse unless the population is reduced in the vast areas affected.