【正确答案】According to the lecture, what the professor is saying is that 20% of the earth's land is desert and the size of desert land will keep increasing in the future. Human activity accounts for a large part of desertification. Our use of natural resources and reckless use of land all contribute to the problem. Of course, such natural forces cause desertification, the corrosion caused by wind, lack of waters, lack of soil like minerals and excessive grazing of animals. In some regions, more water is evaporated than the water coming down as rain. Therefore, due to the small amount of water in an area, the possibility of clouds or rain is small and, in some cases, when the surrounding environment such as mountains cannot stop winds, more water is evaporated from the earth. All these factors are reasons behind the desertification.

【答案解析】[听力原文] Before I start our lecture today, let me explain what desert means. Deserts are a harsh environment with very little rainfall and extreme temperatures. Desert plants like cacti are not abundant; neither is animal life. Deserts are very dry regions with limited plant and animal life. Deserts cover about one-fifth (20 percent) of the earth's land area. Some deserts get very hot during the day and very cold during the night, when temperatures can drop well below freezing. Some deserts, however, are always cold, for example, the Gobi Desert in Asia, and the desert on the continent of Antarctica. Human activities are the main factors triggering desertification. These activities are many and vary by country, society, land use strategies and the local technologies. The impact on human society does not depend solely on its density. Some human activities that can cause desertification are cultivation of soils that are fragile, or exposed to erosion by wind or water, lack of organic or mineral fertilizers, overgrazing by domestic animals. Evaporation is also an important factor contributing to aridity, in some deserts, the amount of water evaporating exceeds the amount of rainfall. Rising air cools and can hold less moisture, producing clouds and precipitation; falling air warms, absorbing moisture. Areas with few clouds, bodies of water and little vegetation absorb the sun's radiation, thus heating the air at the soil surface. More humid areas deflect heat in clouds, water and vegetation, thus remaining cooler. High wind in open country also contributes to evaporation.