话题语篇导读

Because of the development of agriculture，the area of land affected by sand has gradually increased.During the 1950s and 1960s the land affected by sand expanded on average into 1,600 square kilometers per year.By the 1990s，the area had reached 2,500 square kilometers per year.This kind of situation is particularly serious in many regions upwind from Beijing.Fengning City in Hebei Province and Duolun City in Inner Mongolia lie to the north of Beijing，and in the last 50 years the population of these two cities has grown by 2 to 3 times.In order to obtain more resources，the inhabitants have been busy with heavy farming，tree-­cutting and keeping animals.The result is that the soil which has sustained damage，and the sand layer below the earth is now exposed on the ground and the ground is rapidly being controlled by the spread of sand.