Whenever human populations have lived in forest areas, they have always cut down trees which they used for a number of purposes, for housing and ships and served as a source of heating fuel and timber. Growth of cities often meant expansion into forest areas, while even more trees were removed to provide space for agriculture. With the growing demand for paper, vast quantities of trees have also been cut down for paper production. These factors, along with many others, have been contributing to a dangerous phenomenon known as deforestation. In the last 5,000 years, humans have reduced forest from roughly 50 percent of the earth’s land surface to less than 20 percent. Most of this original, or old growth, forest cover is concentrated in three large areas: the Canadian and Alaskan boreal forest, the boreal forest of Russia, and the tropical forest of the northwestern Amazon Basin and the Guyana Shield. These areas comprise almost 70 percent of the world’s remaining original forest cover. In most places, the rate of deforestation is increasing, with the alarming result of 16 million hectares disappearing worldwide every year. Loss of forest does not just mean the decline of natural resources. There are several other factors that make deforestation seriously harmful to both the human and natural worlds. One of them is changes in the global climate. For example, forest clearance is releasing substantial volumes of carbon dioxide (CO2) into the atmosphere as vegetation is burnt or decays. It has been suggested that this is a significant factor in global warming. Moreover, about 10 percent of the world’s tree species are in danger of extinction as a result of deforestation. Deforestation also threatens biological diversity through the destruction of wildlife habitats, which endangers a number of animal species and leads to their potential disappearance. Species are particularly easy to extinction in tropical rainforests because many species have few individuals per unit area, which makes reproduction more difficult. Finally, since forests play an important role in storing water and stabilizing soil, deforestation and the resulting change in land use cause soil erosion (腐蚀) and other forms of land degradation. 72.   The passage discusses all of the following EXCEPT the ________. A. causes of deforestation                                   B. consequences of deforestation C. management of deforestation                      D. rate of deforestation 73.   The word “diversity” in the last paragraph probably means “________”. A. variety                                                                  B. expansion C. development                                                      D. advantage 74.   Why does the author mention fuel and timber in Paragraph 1? A. To explain the rate of deforestation. B. To compare them with housing and ships. C. To show the dangers of deforestation. D. To illustrate the causes of deforestation. 75.   Which of the following sentences summarizes Paragraph 3 best? A. Deforestation threatens biological diversity. B. Deforestation has many harmful consequences. C. Deforestation causes changes in global climate. D. Deforestation should be stopped.