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  3. During the last decade, Val Beasley of the University of Illinois College has led a team wanting to know why the world’s amphibi

During the last decade, Val Beasley of the University of Illinois College has led a team wanting to know why the world’s amphibi

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问题     During the last decade, Val Beasley of the University of Illinois College has led a team wanting to know why the world’s amphibian(两栖动物) populations have been decreasing or riddled with deformed limbs.
    Evidence from his and other teams points to increasing numbers of common parasites as an important cause. However, the problems facing amphibian habitats really pose a distressing example of ecosystems out of balance because of human activity, according to Beasley, a professor of biosciences and executive director of Wildlife and Ecosystem Health. Beasley’s main collaborators, postdoctoral researcher Anna M. Schotthoefer and Rebecca A. Cole, a scientist with the National Wildlife Health Center in Madison, Wis. , say they agree.
    A series of their research projects -— two published in 2003 and another that will appear as a book chapter this year — dramatically refine the data on parasitic activity and argue that physically and chemically induced changes of aquatic habitats are taking a toll.
    "We have to be asking what human activities are contributing to imbalances in these ecosystems to set the stage for more severe infections, " Beasley said. "It’s becoming a serious question of how we can better manage landscapes, streams, wetlands, ponds and lakes. Frogs are among the first animals that young children see in a healthy wild place, but they are not finding them in the same numbers as in past years. "
    Tadpoles (蝌蚪) of many species feed on algae converting the material into the protein, fat and other nutrients that are needed by other creatures higher in the food chain, Beasley said. Amphibians are clearly important players in ecosystem functioning, he added. "The frogs that develop from tadpoles subsequently eat up thousands of insects," he said, "The frogs themselves are also important prey for reptiles, birds and mammals."
    In the study funded by Chicago Zoological Society, Beasley’s team noted that Midwest landowners hollow out their ponds so that deep water prevents the establishment of aquatic plants.
Such a practice allows for easier boating, swimming and fishing, but by eliminating aquatic plants, the habitat becomes structurally barren. Herbicide use also aggravates the problem and produces chemical changes. Plant-derived oxygen is reduced, and herbicides also may have direct toxic effects on tadpoles and other animals, Beasley said. The lack of vegetation may expose tadpoles to more parasites coming from the infected snails, because the tadpoles can’t hide or use the vegetation to brush off the parasites, Schotthoefer said.
    So while the parasites seem to contribute to declines and deformation in frog populations,the challenge is to find management approaches that put balance back into amphibian ecosystems, Beasley said.
The phrase "taking a toll" in the third paragraph probably means
选项 A、causing many deaths.
B、changing frogs’ behaviors.
C、accounting for human activities.
D、posing a threat to parasites.
答案A
解析 第四段对第三段末句进行了解释：我们必须要问，哪些人类活动的后果导致这些生态系统的失衡，造成了两栖动物更加严重的感染…青蛙是孩子们在健康的野外环境中首先看到的动物之一，但现在，孩子们找不到像过去那么多的青蛙了。由物理因素和化学因素诱发水生栖息地发生变化的结果是“两栖动物更易感染疾病”。具体事例就是“青蛙数量的减少”，即青蛙大量死亡，故[A]为答案。
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