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. A. Desertification is a significant problem because it is so hard to reverse and affects large areas of land and great numbers of people. B. Slowing down the process of desertification is difficult because of population growth that has spread over large areas of land. C. 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. D. Desertification is extremely hard to reverse unless the population is reduced in the vast areas affected.