Biologists have noted reproductive abnormalities in fish that are immediately downstream of paper mills. One possible cause is dioxin, which paper mills release daily and which can alter the concentration of hormones in fish. However, dioxin is unlikely to be the cause, since the fish recover normal hormone concentrations relatively quickly during occasional mill shutdowns and dioxin decomposes very slowly in the environment. Which one of the following statements, if true, most seriously weakens the argument? (A) Some of the studies that show that fish recover quickly during shutdowns were funded by paper manufacturers. (B) The rate at which dioxin decomposes varies depending on the conditions to which it is exposed. (C) Normal river currents carry the dioxin present in the river far downstream in a few hours. (D) Some of the fish did not recover rapidly from the physiological changes that were induced by the changes in hormone concentrations. (E) The connection between hormone concentrations and reproductive abnormalities is not thoroughly understood.
Why C not D? I did not figure out the "trick" to this question however.
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发表于 2006-5-30 03:47:00
