A.I. Explanation:

Option 1: "Desertification is a significant problem because it is so hard to reverse and affects large areas of land and great numbers of people."

Explanation: This option accurately captures the main idea of the highlighted sentence, which emphasizes the seriousness of desertification due to its extensive impact on land and people, as well as the difficulty in reversing the process. The phrase 'significant problem' aligns with the text's description of 'extreme seriousness,' and the mention of 'hard to reverse' reflects the text's assertion about the challenges in reversing desertification.

Option 2: "Slowing down the process of desertification is difficult because of population growth that has spread over large areas of land."

Explanation: This option incorrectly attributes the difficulty of slowing desertification to population growth, which is not mentioned in the text. The text focuses on the challenges of reversing desertification due to erosion and the time required for new soil formation, rather than population growth. Thus, this option misrepresents the cause of the difficulty.

Option 3: "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."

Explanation: This option introduces the idea that a solution requires involvement from large numbers of people, which is not supported by the text. The text discusses the seriousness of desertification and the difficulty of reversing it, but it does not suggest that a collective effort is necessary for a solution. Therefore, this option adds information that is not present in the original text.

Option 4: "Desertification is extremely hard to reverse unless the population is reduced in the vast areas affected."

Explanation: This option incorrectly implies that reducing the population is a prerequisite for reversing desertification, which is not stated in the text. The text emphasizes the difficulty of reversing desertification due to soil erosion and the time required for soil formation, without mentioning population reduction as a factor. Thus, this option misinterprets the text's message.