托福阅读否定事实信息题范例6

　　DESERT FORMATION

　　Paragraph 9: The final major human cause of desertification is soil salinization resulting from over irrigation. Excess water from irrigation sinks down into the water table. If no drainage system exists, the water table rises, bringing dissolved salts to the surface. The water evaporates and the salts are left behind, creating a white crustal layer that prevents air and water from reaching the underlying soil.

　　All of the following are mentioned in the passage as contributing to desertification EXCEPT

　　Soil erosion

　　Global warming

　　Insufficient irrigation

　　The raising of livestock