探讨一逻辑单题

Biologists have noted reproductive abnormalities in fish that are immediately downstream of paper mills. One possilbe 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 the following statements, if true, most seriously weaken the argument?

A. some of the studies that show that fish recover quickly during shutdowns were funded by paper manufacurers.

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.

请问，正确答案是什么？每错误选项是如何错的？本题的设计原理是什么？

D项是怎么错的呢？

我认为这个题干的issue 是dioxin 与hormone concentration 的关系。如果这一点断定得比较准的话，可以缩小答案范围到B和C。因为ADE与这个核心issue 的关系都比较远。具体解释如下：A 是指出研究资金来源，但你不能assume这些研究者为了paper mills 的利益而有意颠倒黑白。D 虽指出some fish 和其他的fish recover的时间长短不同，但如果要weaken 这个argument，还需要指出各自的比例，或者哪些fish更有代表性。E hormone concentration 和 reproductivity 的关系即使不清楚，也不影响argument在核心issue 的观点。下面解释为什么C最好。the argument to be weakened is 在paper mills shutdown 时，环境中的dioxin并未影响fish 的hormone concentration, 进而否定它们之间的因果关系。C 则指出fish that are immediately downstream 实质上要历经 a few hours 的 exposure to dioxin.在shutdown时，残存的dioxin是不能与之相比的。（任何的毒物要起作用都需要一定的浓度和接触时间）再说B，在dioxin 的降解和shutdown之间关系不明的情况下，无法判断与argument的相关性。

题目让我们削弱的argument是用“有A(dioxin），而没有B（reproductive abnormalty）”的事实，来否定基于“有A，有B”这个事实的AB有因果关系。答案C的正确在于指出argument中的A（环境中残存） 并非真有（hours exposure）。