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.

[spoiler]OA: C[/spoiler]

conclusion: dioxin is unlikely to be the cause of change in hormone concentration in fish
to weaken we must show that dioxin is likely to be the cause

A. out of scope
B. out of scope

C. correct. dioxin is likely to be the cause as normal currents cause dioxin to reach downstream in a few hours

D. out of scope. physiological changes due to change in hormone concentration never discussed

E. doesnt weaken

[spoiler](C)[/spoiler]

scoobydooby wrote:conclusion: dioxin is unlikely to be the cause of change in hormone concentration in fish
to weaken we must show that dioxin is likely to be the cause

A. out of scope
B. out of scope

C. correct. dioxin is likely to be the cause as normal currents cause dioxin to reach downstream in a few hours

D. out of scope. physiological changes due to change in hormone concentration never discussed

E. doesnt weaken

Can you throw some more light? I was between C) and D). Am unable to decide why C)?

(D) mentions about some fish. I guess this is one of the traps GMAT uses a lot of times.

If (C) were to be true, all fish would be affected.

I understand the some part.

However, just because "Normal river currents carry the dioxin present in the river far downstream in a few hours" doesn't mean that it is cause.

The currents carry dioxin downstream(as per C) and the fish are there downstream (as per argument)

scoobydooby wrote:conclusion: dioxin is unlikely to be the cause of change in hormone concentration in fish
to weaken we must show that dioxin is likely to be the cause

A. out of scope
B. out of scope

C. correct. dioxin is likely to be the cause as normal currents cause dioxin to reach downstream in a few hours

D. out of scope. physiological changes due to change in hormone concentration never discussed

E. doesnt weaken

Good Question turbojet.

I think D is strengthening the argument by eliminating the alternate cause
Any thoughts?

schumi_gmat wrote:

scoobydooby wrote:conclusion: dioxin is unlikely to be the cause of change in hormone concentration in fish
to weaken we must show that dioxin is likely to be the cause

A. out of scope
B. out of scope

C. correct. dioxin is likely to be the cause as normal currents cause dioxin to reach downstream in a few hours

D. out of scope. physiological changes due to change in hormone concentration never discussed

E. doesnt weaken

Good Question turbojet.

I think D is strengthening the argument by eliminating the alternate cause
Any thoughts?

I agree too

hey schumi and vinaynp,
the conclusion is : dioxin not likey to cause change in hormone concentration in fish.

D says some fish did not recover from some effect of the hormone concentration. it talks of the result of the change in hormone concentration, not the cause of change in hormone concentration.
we need something that would affect the conclusion. D doesnt affect it.

C comes closest of all the choices- says the dioxins released daily are carried by normal currents downstream where fish are found. it makes dioxins 'likely' (not certainly) to cause change in hormone concentration

The stimulus is talking about the fish immediately downstream of the paper mills. The answer choice C says the 'far down stream'. wouldn't this strengthen the conclusion? can any one explain?

First of all spot the conclusion and the premise

Conclusion: Dioxin is not the cause for reproductive abnormalities in fish

Stated Central Premise: Even if hormones change, the fish are able to restore them quickly

Logic in the argument: X (Dioxin) leads to Y(Change in hormones) leads to Z (reproductive abnormalities)

Can you spot the flaw in author's logic here?
The premise of the author is assuming that X has led to Y. The author is thereby defending Y and saying that Z cannot occur.

Now try to find an answer choice that fixes the author's logical flaw.
We can do this if we can find an answer choice wherein X does not lead to Z i.e. Dioxin does not lead to reproductive abnormalities in fish

Careful: The trick words in this qs are upstream and far upstream

Answer C tells us there is no dioxin upstream. So fish cannot be affected by dioxin. (Note dioxin decomposes slowly) Hence fish upstream have reproductive abnormalities due to some other cause

Tip: In weakening qs, if one cause is given to prove a conclusion, try to find the possibility of another cause that can also prove the conclusion.Thus, you can weaken the argument by finding more than 1 cause for a conclusion

Elimination Reasons:

A: Is supporting the central premise

B: Irrelevant/ Out of scope (We are concerned only with dioxin, reproductive abnormality and fishes upstream)

D: Out of scope. ( We do not know whether physiological changes occurred due to hormone imbalances)Also it talks of Y and Z relationship which the author is anyways talking about. Nothing new.

E: This looks to be a preferred choice. However this also talks about Y and Z relationship. However the focus of the conclusion is on dioxin and repr abnormalities


Hope this helps clear some doubts!!!
Cheers!!
Turbo Jet

krazy800 wrote:The stimulus is talking about the fish immediately downstream of the paper mills. The answer choice C says the 'far down stream'. wouldn't this strengthen the conclusion? can any one explain?

I think the same, C strengthens the conclusion eliminating the possibility that dioxin be the cause. I'm sure I'm missing something here

It seems to me that the right answer is C