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　　The following is a letter to the editor of an environmental magazine.

　　"In 1975 a wildlife census found that there were seven species of amphibians in Xanadu National Park, with abundant numbers of each species. However, in 2002 only four species of amphibians were observed in the park, and the numbers of each species were drastically reduced. There has been a substantial decline in the numbers of amphibians worldwide, and global pollution of water and air is clearly implicated. The decline of amphibians in Xanadu National Park, however, almost certainly has a different cause: in 1975, trout—which are known to eat amphibian eggs—were introduced into the park."

　　Write a response in which you discuss what specific evidence is needed to evaluate the argument and explain how the evidence would weaken or strengthen the argument.

　　The following is a letter to the editor of an environmental magazine.

　　"Two studies of amphibians in Xanadu National Park confirm a significant decline in the numbers of amphibians. In 1975 there were seven species of amphibians in the park, and there were abundant numbers of each species. However, in 2002 only four species of amphibians were observed in the park, and the numbers of each species were drastically reduced. One proposed explanation is that the decline was caused by the introduction of trout into the park's waters, which began in 1975. (Trout are known to eat amphibian eggs.)"

　　Write a response in which you discuss one or more alternative explanations that could rival the proposed explanation and explain how your explanation(s) can plausibly account for the facts presented in the argument.

　　新东方GRE教师宁立波老师的思路梳理：

　　1. amphibian may die for reasons other than trout

　　2. during 1975 to 2002, amphibian as well as trout may have both increased for decades and together in 2000, for example, declined due to water and air pollution

　　3. observation is not as accurate as census

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