Biologists have noted reproductive abnormalities in fish that are imme

Context:
We are talking about the fish immediately downstream of a paper mill. They are facing reproductive issues.
Possible Cause - Dioxin which mills release every day. It alters fish hormones

Premises:
Fish recover normal hormone concentrations relatively quickly during occasional mill shutdowns
Dioxin decomposes very slowly in the environment.

The author is saying that when the mill shutdown for a few days (no new dioxin coming in the water) and old dioxin is still present (because it decomposes very slowly in the environment - in the water), fish recover normal hormone concentrations. It means that something else might be responsible for the physical issues the fish are facing because even with the dioxin present (because it decomposes very slowly), the fish regain the normal balance. It means the mills might be releasing some other chemical affecting the fish or something else may be responsible.

Conclusion:
Dioxin is unlikely to be the cause of reproductive issues.

We have to weaken the conclusion.

(A) Some of the studies that show that fish recover quickly during shutdowns were funded by paper manufacturers.

'Some' studies were funded by paper manufacturers, not all. Perhaps some were independent? We can't say. Not enough info.

(B) The rate at which dioxin decomposes varies depending on the conditions to which it is exposed.

We know that dioxin decomposes very slowly in the environment. The rate of decomposition could vary say 1 unit per million per day or 2 units per million per day or 5 units per million per day depending on say water temperature etc but the rate will be very low. So the old dioxin should be present in the water when the mill is shut down and hence it should affect the fish. It doesn't weaken our argument.

Saupayan - We are talking about the rate at which dioxin decomposes in the environment. When it is taken in by the fish, how it impacts the fish and what chemicals it converts into etc is irrelevant. We know that it causes hormonal imbalance when it goes inside the fish and that is all. The rate of decomposition is related to the rate at which it breaks down while present in the water.
The question is fine and could very well be an official question.

(C) Normal river currents carry the dioxin present in the river far downstream in a few hours.

This tells us that the old dioxin does not stay in the water because it is carried far away by the river. Then it is possible that the dioxin is the one affecting the fish. The reason the fish recover when the mills shut down for a few days is that no new dioxin is coming into the water.
Correct.

(D) Some of the fish did not recover rapidly from the physiological changes that were induced by the changes in hormone concentrations.

It says 'some' while we are talking about the fish in general. Because some did not recover rapidly, we cannot say that dioxin may have been responsible.

(E) The connection between hormone concentrations and reproductive abnormalities is not thoroughly understood.

It doesn't give us any meaningful information.

Answer (C)