10．Birds enjoy a relatively slow rate of extinction but a new study suggests that rate might be severely underestimated（低估）．Even worse，if human actions continue，bird extinction rates could skyrocket and 12percent of the known bird species（物种） could be dying out by the end of the century．
Presently there are 10，000known bird species-most identified after 1850-and an estimated 130of those have become extinct since 1500，setting the extinction rate at about one species every four years．
But according to Stuart Pimm of Duke University，this rate fails to take into account three key points：The continual identification of extinct bird species from fossil remains； numerous"missing"species not yet declared extinct； and the fact that present extinction rates were not calculated using the proper baseline date for when the species was first described．
Taking these points into consideration，the extinction rate is closer to one bird species per year，says Pimm，the leading author of the study．And the rate could be three times as high as that if not for recent bird preservation efforts．
In previous centuries，bird extinctions took place mainly on islands as Polynesian peoples expanded into the Pacific or Europeans took over the Americas，wiping out birds along the way．
In recent years，scientists are seeing an increased number of extinctions on continents，again because of human activities．Habitat destruction，introduction of invasive species，and climate change combine to harm many bird species．Before human influence，the estimated rate of bird extinctions would have been only one species per hundred years，researchers estimate．
Bird extinction rates are slower than for most animals，mainly because humans do more harm to other species and people take special efforts to protect birds．Still，if the present trends continue，the researchers estimate that the bird extinction rate will continue to climb to as many as 10species per year，
47．The underlined word"skyrocket"in the first paragraph meansC
A．fly high
B．appear soon
C．increase quickly
D．change regularly
48．According to Pimm，the extinction rate of birds isB．
A．one species per hundred years
B．one species every year
C．three species per year
D．ten species every year
49．What can we infer from the fifth and sixth paragraphs？D
A．Humans have started destroying bird habitat in recent years．
B．Humans are doing more harm to animals than to birds．
C．Humans have made no effort to protect birds．
D．Humans are responsible for the bird extinctions．
50．What might be the best title for the passage？C
A．Humans'Fighting against Bird Extinction
B．Humans'Responsibilities for Bird Protection
C．Bird Extinction Rates Far Worse Than Realized
D．Bird Extinction Occurring on Islands and Continents．
分析 本文讲到研究表明以前鸟类灭绝率被低估了，如果不采取措施，灭绝率会飞速上升，随后用研究的事实及结论来告诉我们，由于人类的影响，鸟类灭绝的速度在增加．
解答 CBDC
47．C 词义猜测题 根据前一句Birds enjoy a relatively slow rate of extinction but a new study suggests that rate might be severely underestimated（低估）．可知研究表明以前鸟类灭绝率被低估了，如果不采取措施，灭绝率会飞速上升，故skyrocket的意思是迅速增加，故答案为C
48．B 细节理解题 根据Taking these points into consideration，the extinction rate is closer to one bird species per year，says Pimm，the leading author of the study．可知，每年会灭绝一种鸟类，故答案为B
49．D 推理判断题 根据Before human influence，the estimated rate of bird extinctions would have been only one species per hundred years，researchers estimate．可知人类应该为鸟类的大量灭绝负责，故答案为D
50．C 主旨大意题 根据第一段提到研究表明以前的灭绝率被低估了，如果不采取措施，灭绝率会飞速上升，随后用研究的事实及结论来告诉我们，由于人类的影响，鸟类灭绝的速度在增加，故标题可拟为Bird Extinction Rates Far Worse Than Realized．
点评 阅读理解题测试考生在阅读基础上的逻辑推理能力，要求考生根据文章所述事件的逻辑关系，对未说明的趋势或结局作出合理的推断；或根据作者所阐述的观点理论，对文章未涉及的现象、事例给以解释．考生首先要仔细阅读短文，完整了解信息，准确把握作者观点．
