Critical Reasoning

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.