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

I WONDER IF THE TEMP. ACTUALLY DECREASES OR THE RANGE OF TEMP DOES. TRICKY QUESTION.

IMO C. the scientists say SHARP RISE in temperature. this can happen only when there is a vast difference between the highest and lowest. best explained by C

I picked the answer A:

The conclusion is: [highlight]Scientists have hypothesized that sharply rising water temperatures must be involved in signaling the native species to begin the reproductive cycle.[/highlight]

A. This presents a strong reason to support their hypothesis. You are essentially performing a sensitivity test - you are just changing the temperature range and seeing how the fish react. All the other variables have been kept constant (ie. same fish, same river etc.). Therefore, if the same fish can reproduce in the same river with only a change in the temperature range, that is proof that the dam has changed the reproductive cycle.

B. First, I believe that this point is irrelevant because we are only concerned about the relationship between reproductive cycle in the river below the dam and the temperature of the river. Also, this answer choice could potentially point to another reason why reproductive cycles happened before the dam was built - in this case it would weaken the argument. In either case, this answer is incorrect.

C. This answer choice only gives you 2 specific temperatures. The first question I had was "where were temperatures recorded?" There is no explanation of that in this answer choice, so the temperature readings could actually have been taken before the dam - that would be irrelevant to this question since we are only concerned with the river after the dam. Also, we are only concerned with the RANGE of temperatures and not the temperature itself.

D. This could provide an alternate explanation as to why the fish are not reproducing. Therefore, this answer choice would actually weaken the argument.

E. What does this even tell us? This answer choice is irrelevant.

Second that. Is there a catch between range of temperature and actual increase/decrease in temperature?

Cheers
J

There is no need to wonder over this one..even if the range decrease is true by the measured temps...it is something that the premise states explicitly..So it is just repeating an info that we already know..A dangerous trap in such questions

You might call it a trap though..coz the temperature drop decrease does not imply lower range..what if there was an increase in global temperatures altogether?

Conclusion: Sharply rising temp signals fish to begin reproductive cycle
Premise: Range decreased from 50 to 6

Premise( say temperature increased from 10 to 60 , rather than from 10 to 16) leads strongly to conclusion. Correct answer will provide examples for the hypothesis.
C just mentions lowest temperatures, nothing new is provided. Lowest recorded before 34, highest 84. Lowest recorded before 43, highest 49. No new evidence. Correct choice for a strengthen question will go slightly out of scope.

we do not need formal logic to do this problem.
A cause B
no A cause no B because if A exist,we have B. this is formal logic which I dont know and dont need to know
choice A in fact is another facet of arguement. choice A is a strengthener.

Data:
- Dam built (lets call it X) , Fish did not reproduce enough( Lets call it Y)
- Average annual temp decreased (Lets call it Z)

Notice that we have correlation implied between:
- X & Y
- Z & X
- Z & Y

Conclusion:
- In order to signal (I assume that natures signals) -> the water temp is rising.
--------------------
Assumptions:
- Nature signals to fish to reproduce by rising the water temp
- High enough temp -> fish reproduce

[V] - A - This indeed strengthen one of the assumptions.
[X] - B - This might explain why the overall fish population decreased, by telling us that an essential factor for breeding was not available, but it got nothing with conclusion (temperature is not mentioned).
[X] - C - This choice have no effect. In addition, a signal data point cannot indicate a general trend.
[X] - D - This might explain why the overall fish population decreased by telling us that critical factors for the fish survival was scarce, but it got nothing to do with the conclusion.
[X] - E - Clearly out of scope.

that's a Strenghten question: we need to find an answer choice saying that raising temprature would be helpful in increasing the population

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.
This supports what scientists have hypothesized.

(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.
This option suggests that after the dam was built, Emerald river could not overflow and thus no backwaters created, thus it provides another cause for the breeding inadequacy of the native fish species of the Emerald river. It weakens.
Choice B undermines the hypothesis by suggesting a completely different 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.
The argument is talking about the annual range of temperature, but this option is talking about the lowest recorded temperature. Therefore, just on the basis of lowest recorded temperature we cannot comment on the hypothesis of the scientists.

(D) Non-native 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 option D relates a development after the native species began to decline, it does not bear on the hypothesis, which concerns the decline’s original cause;

(E) Five of the fish species native to the Emerald River are not native to any other river in North America.
Irrelevant. This option statement has no impact on the above scientist's hypothesis.

Can someone explain option C in more detail

Correct choice is 'A' - The construction of a dam has significantly reduced the range of water temperatures in the river below the dam. Scientists have implicated this change in the failure of native fish species to reproduce adequately. Hence, statement 'A' strengthen Scientists' hypothesis.

Hello GMATNinja

Can you explain A vs C?



Is C wrong because it doesn't mention that these lowest temperatures were of the river below the dam? It just mentions the lowest temperatures in general of the entire river



Although Option A mentions that the range is approximately same, we don't know whether the side stream has the exact same temperatures. Let me put down an example to make it clear
1. Temperature range of the river below the dam before the dam was built : 60 degree to 10 Degree
2. Temperature range of the river below the dam after the dam was built : 26 degree to 20 degree
3. Temperature range of the side streams below the dam after the dam was built : 10 degree to -40 degree

If you see #3 - the range is same but reproduction may depend on the rising temperatures that were attained in case #1. So may weaken the conclusion or strengthen

I think there is a very small difference between A and C. Let me know your thoughts

Here, we are supposed to strengthen the hypothesis - Sharply Rising Water temperatures trigger reproduction cycle.

However, Option A just highlights that reproduction is happening because the temperature range is 50 degrees. It doesn't imply that it is the SHARPLY RISING TEMPERATURE that is causing the reproduction. It may be the SHARP REDUCTION in temperature that triggers the reproduction cycle, or it may be a particular temperature which triggers the reproduction(which the current 6 degrees range is not allowing the water to reach).

I marked C, because it atleast talks about the water temperatures which is pertinent to the hypothesis.

DavidTutorexamPAL MartyTargetTestPrep VeritasKarishma nightblade354 GMATNinjaTwo gmat1393

I faltered in understanding the argument and request your two cents.

Start with the question stem:



So, I need to strengthen the scientists's hypothesis which is:
sharply rising water temperatures must be involved in signaling the native species to begin the reproductive cycle.
Or,
sharply rise in water temperatures led to fish begin their reproductive cycle.



But in above argument, final temperature is 6 deg and OE suggests that fish reproduce ONLY in temperatures at 50 deg C.


Does this mean building of dam led to reduction in reproduction of fish species?

Hey adkikani,

I think you may have missed the word 'range'.
The 'final temperature' is not 6 deg, rather the 'range of final tempratures' = max temp - min temp = 6deg.
So basically the hypothesis is that sharply increasing temps are required for reproduction, and the support is that now that the dam is built the range of temps is very small (so sharply increasing temp is an impossibility).
(A) supports this by saying that once you 'fix' the range of temperatures, you also 'fix' the reproduction.

I got confused and picked C; C is the contender.

Before Range: 50
Before the dam built Lowest temp: 34
then Highest must be: 50+34= 84

After Range: 6
After Lowest Temp: 43
then Highest must be: 49

To get the same condition as there were before the Dam we need to increase the temp.

but if you re-read the conclusion we need to find the option to support that the range is a factor; Hence Option A.

Dear Experts
Can you please explain all the answer choices in detial. Also why specifically C iss wrong? I also think that such a question may not come on the GMAT due to the ambiguity in the language. Do you also feel so.

Thanks.