Biologist 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?
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.
Why is it C and not D. If the fish don't recover, from the changes in hormone imbalences doesn't this weaken the argument?