How to identify the "argument"

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? I Identified the argument structure as follows: Counter P - 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. C - However, dioxin is unlikely to be the cause P - since the fish recover normal hormone concentrations relatively quickly during occasional mill shutdowns and dioxin decomposes very slowly in the environment. So essentially the “argument” that the question is referring to is: “However, dioxin is unlikely to be the cause, since the fish recover normal hormone concentrations relatively quickly and dioxin decomposes very slowly in the environment.” How is (C) “normal river currents carry the dioxin present in the river far downstream in a few hours” the right answer? The question asks us to identify what weakens the argument while (C) appears to strengthen the argument. I have reviewed prior lectures and still cannot find an example in them to determine where I went wrong with this question. If this is covered in material after Diagnostic 1, could you please tell me where so that I can learn the concept necessary to approach this question? Thank you!