Critical Reasoning

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.

Please post your answers with the discussions. I am at odds with the OA.

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.
--> 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.
--> doesnt touch the conclusion

(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.
--> if it does something it weakens the conclusion

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

IMO a

Hi Thiagaraj,
Plz comment on my approach below:
Premise: 0 of 8 fish species native to ER reproduce adequately. Water temperature in river 50 degree -> 6 degree
Conclusion: high degree involve in signaling native species to reproduce.
Question: Strengthen -> high degree lead to reproduce.
(A) Annual temperature range remains 50 degrees -> continue to reproduce -> Correct
(B) doesn't talk to temperature, only reprocedure
(C) Only talk about temperature
(D) Nonnative species of fish -> out of scope. We don't talk about nonnative here
(E) doesn't talk to temperature & reproduce.
Therefore, correct answer is (A)

A) is the right option. Note that the fish were able to reproduce on the other side of the "same river" which means that there is nothing wrong with the waters. Hence that leaves temperature as the only factor. Besides, the rest of the options are far worse.

scholardream wrote:Hi Thiagaraj,
Plz comment on my approach below:
Premise: 0 of 8 fish species native to ER reproduce adequately. Water temperature in river 50 degree -> 6 degree
Conclusion: high degree involve in signaling native species to reproduce.
Question: Strengthen -> high degree lead to reproduce.
(A) Annual temperature range remains 50 degrees -> continue to reproduce -> Correct
(B) doesn't talk to temperature, only reprocedure
(C) Only talk about temperature
(D) Nonnative species of fish -> out of scope. We don't talk about nonnative here
(E) doesn't talk to temperature & reproduce.
Therefore, correct answer is (A)

Thanks all. Thing is I was and still am not sure on the stimulus. There is one part of the question that particularly bugs me. This one
"Since the dam reduced the annual range of water temperature in the river below the dam from 50 degrees to 6 degrees"

I had taken it to mean the water temperature ranges from 50 to somewhere around 43 degrees which is approximately six less than 50. But before the dam was built, the temperature ranges had been more, much more, so that it can be called 'sharply rising water temperatures'.

Please explain me this one sentence. With it in mind, I cannot accept A as the OA.

Before the dam, the range was 50; perhaps the summer high was 85 and the winter low was 35. That would qualify as a sharp increase in temperature.

After, the range is 6; perhaps 35 to 41. Those are practically the same temperature, so that sharp increase is no longer present.

Bill

Thanks Bill. That clears it up.