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Two decades after the Emerald River Dam was built, none of

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Two decades after the Emerald River Dam was built, none of [#permalink] New post 20 Jun 2008, 01:56
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A
B
C
D
E

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Two decades after the Emerald River Dam was built, none of the eight fish species native to the Emerald
River was still reproducing adequately in the river below the dam. Since the dam reduced the annual range of
water temperature in the river below the dam from 50 degrees to 6 degrees, scientists have hypothesized that
sharply rising water temperatures must be involved in signaling the native species to begin the reproductive
cycle.
Which of the following statements, if true, would most strengthen the scientists’ hypothesis?
(A) The native fish species were still able to reproduce only in side streams of the river below the dam where the
annual temperature range remains approximately 50 degrees.
(B) Before the dam was built, the Emerald River annually overflowed its banks, creating backwaters that were
critical breeding areas for the native species of fish.
(C) The lowest recorded temperature of the Emerald River before the dam was built was 34 degrees, whereas
the lowest recorded temperature of the river after the dam was built has been 43 degrees.
(D)Nonnative species of fish, introduced into the Emerald River after the dam was built, have begun competing
with the declining native fish species for food and space.
(E) Five of the fish species native to the Emerald River are not native to any other river in North America.
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Re: CR: UNDER THE DAM [#permalink] New post 20 Jun 2008, 02:06
IMO A

OA ??
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Re: CR: UNDER THE DAM [#permalink] New post 20 Jun 2008, 02:08
IMO A

The native fish species were still able to reproduce only in side streams of the river below the dam where the
annual temperature range remains approximately 50 degrees.


The statement clearly shows that there is a relation between temperature and reproductive cycle of native fish.

All other options fail to establish this relation.
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Re: CR: UNDER THE DAM [#permalink] New post 20 Jun 2008, 02:59
Clear A
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Re: CR: UNDER THE DAM [#permalink] New post 20 Jun 2008, 03:25
A must be the OA
lexis wrote:
Two decades after the Emerald River Dam was built, none of the eight fish species native to the Emerald
River was still reproducing adequately in the river below the dam. Since the dam reduced the annual range of
water temperature in the river below the dam from 50 degrees to 6 degrees, scientists have hypothesized that
sharply rising water temperatures must be involved in signaling the native species to begin the reproductive
cycle.
Which of the following statements, if true, would most strengthen the scientists’ hypothesis?
(A) The native fish species were still able to reproduce only in side streams of the river below the dam where the
annual temperature range remains approximately 50 degrees.
(B) Before the dam was built, the Emerald River annually overflowed its banks, creating backwaters that were
critical breeding areas for the native species of fish.
(C) The lowest recorded temperature of the Emerald River before the dam was built was 34 degrees, whereas
the lowest recorded temperature of the river after the dam was built has been 43 degrees.
(D)Nonnative species of fish, introduced into the Emerald River after the dam was built, have begun competing
with the declining native fish species for food and space.
(E) Five of the fish species native to the Emerald River are not native to any other river in North America.
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Re: CR: UNDER THE DAM [#permalink] New post 20 Jun 2008, 09:30
Many people say only "their choice" without any explanation. GMATClub is place for GMAT test takers discuss, so your contributions are highly appreciated.
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Re: CR: UNDER THE DAM [#permalink] New post 20 Jun 2008, 09:44
Quote:
Since the dam reduced the annual range of
water temperature in the river below the dam from 50 degrees to 6 degrees, scientists have hypothesized that
sharply rising water temperatures must be involved in signaling the native species to begin the reproductive
cycle.


Which of the following statements, if true, would most strengthen ]the scientists’ hypothesis?

Thus we need to look for an option that shows the reduced range as the culprit

(A) The native fish species were still able to reproduce only in side streams of the river below the dam where the annual temperature range remains approximately 50 degrees.-> thus it supports the conclusion
(B) Before the dam was built, the Emerald River annually overflowed its banks, creating backwaters that were critical breeding areas for the native species of fish.-> could be true but this DOES NOT strengthen the conclusion
(C) The lowest recorded temperature of the Emerald River before the dam was built was 34 degrees, whereas the lowest recorded temperature of the river after the dam was built has been 43 degrees. -> we dont know it's affect on the range since maximum temp could be anything
(D)Nonnative species of fish, introduced into the Emerald River after the dam was built, have begun competing
with the declining native fish species for food and space.

(E) Five of the fish species native to the Emerald River are not native to any other river in North America.
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Re: CR: UNDER THE DAM [#permalink] New post 20 Jun 2008, 09:59
I agree with lexis. My pick is A.
Reason..conclusion..because temp range has been narrowed so native fish are not reproducing. We need some argument to strength our conclusion, clearly only A does strength the conclusion.
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Re: CR: UNDER THE DAM [#permalink] New post 20 Jun 2008, 19:15
(A) The native fish species were still able to reproduce only in side streams of the river below the dam where the
annual temperature range remains approximately 50 degrees.
- seems correct
(B) Before the dam was built, the Emerald River annually overflowed its banks, creating backwaters that were
critical breeding areas for the native species of fish.
- irrelevant
(C) The lowest recorded temperature of the Emerald River before the dam was built was 34 degrees, whereas
the lowest recorded temperature of the river after the dam was built has been 43 degrees.
- irrelevant
(D)Nonnative species of fish, introduced into the Emerald River after the dam was built, have begun competing
with the declining native fish species for food and space.
- irrelevant
(E) Five of the fish species native to the Emerald River are not native to any other river in North America.
- irrelevant
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Re: CR: UNDER THE DAM [#permalink] New post 22 Jun 2008, 21:20
Frankly, I think A weaken then conclusion.
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Re: CR: UNDER THE DAM [#permalink] New post 23 Jun 2008, 10:30
Since the dam reduced the annual range of
water temperature in the river below the dam from 50 degrees to 6 degrees, scientists have hypothesized that
sharply rising water temperatures must be involved in signaling the native species to begin the reproductive
cycle.

A) The native fish species were still able to reproduce only in side streams of the river below the dam where the
annual temperature range remains approximately 50 degrees.
- > A correctly states that the native species of fish reproduction is subject to temperature hence supports
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Re: CR: UNDER THE DAM [#permalink] New post 23 Jun 2008, 11:27
lexis wrote:
Frankly, I think A weaken then conclusion.


A most strengthens the hypothesis. Lets recap:

Hypothesis: Sharp rises in temperature cause increased reproduction levels
Condition: Range of temperatures used to be "50". Current range is "6". Lets say that:

Pre-dam: temperatures ranged between 10 and 60 degrees (high reproduction)
Post-dam: temperatures now range between, say, 30 and 36 degrees (low reproduction)

The scientists are hypothesizing that the reproduction cycle kicks in once temperatures increase. Naturally, there is a higher chance of a noticeable increase in temperature if annual temps vary between 10 and 60 degrees, than if temps vary between 30 and 36 degrees

Which MOST strengthens the hypothesis?

(A) The native fish species were still able to reproduce only in side streams of the river below the dam where the
annual temperature range remains approximately 50 degrees.


If there is evidence that the fish reproduced in the area with a 50 degree range in temperature, this supports the scientists hypothesis (no reproduction in other areas where the range was much smaller)

(B) Before the dam was built, the Emerald River annually overflowed its banks, creating backwaters that were
critical breeding areas for the native species of fish.

May be true, but has nothing to do with temperature ranges and hence doesn't support the hypothesis.

(C) The lowest recorded temperature of the Emerald River before the dam was built was 34 degrees, whereas
the lowest recorded temperature of the river after the dam was built has been 43 degrees.

Irrelevant because the issue here is not lowest or highest temperature, it is the range of the temperatures. Pre-dam, temperatures could have ranged from 34 to 90, post dam from 43 to 60. It does not address temperature ranges and hence does not support the hypothesis.

(D)Nonnative species of fish, introduced into the Emerald River after the dam was built, have begun competing
with the declining native fish species for food and space.

Could be a cause of a decline in reproduction. BUT it doesnt support the hypothesis

(E) Five of the fish species native to the Emerald River are not native to any other river in North America.

I don't see how this could be relevant in any way! :P



Hope this helped!
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Re: CR: UNDER THE DAM [#permalink] New post 23 Jun 2008, 11:33
What is OA btw?
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Re: CR: UNDER THE DAM [#permalink] New post 23 Jun 2008, 19:06
Premise
-none of the eight fish species native to the Emerald
River was still reproducing adequately in the river below the dam.
-Since the dam reduced the water temperature in the river below the dam from 50 degrees to 6 degrees

Conclusion
- sharply rising water temperatures must be involved in signaling the native species to begin the reproductive
cycle.

Assumption- warmer temp play a role in the reproduction activity of teh fishes



(A) The native fish species were still able to reproduce only in side streams of the river below the dam where the
annual temperature range remains approximately 50 degrees.
>>supports the assumption

(B) Before the dam was built, the Emerald River annually overflowed its banks, creating backwaters that were
critical breeding areas for the native species of fish.
>>> irrelevant

(C) The lowest recorded temperature of the Emerald River before the dam was built was 34 degrees, whereas
the lowest recorded temperature of the river after the dam was built has been 43 degrees.
>>>irrelevant

(D)Nonnative species of fish, introduced into the Emerald River after the dam was built, have begun competing
with the declining native fish species for food and space.
>>> this should affect the population size.Irrelevant.

(E) Five of the fish species native to the Emerald River are not native to any other river in North America
>>irrelevant
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Re: CR: UNDER THE DAM [#permalink] New post 28 Jun 2008, 11:26
Thanks! OA is A.
Re: CR: UNDER THE DAM   [#permalink] 28 Jun 2008, 11:26
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