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tracyyahoo: Master | Next Rank: 500 Posts; Posts: 147; Joined: Tue Jun 14, 2011 7:03 am; Thanked: 3 times

by tracyyahoo » Tue Oct 04, 2011 7:31 am

In the late 1980's, the population of sea otters in the North Pacific Ocean began to decline. Of the two plausible explanations for the decline-increased predation by killer whales or disease-disease is the more likely. After all, a concurrent sharp decline in the populations of seals and sea lions was almost certainly caused by a pollution-related disease, which could have spread to sea otters, whereas the population of killer whales did not change noticeably.

Which of the following, if true, most seriously weakens the reasoning?

A. Killer whales in the North Pacific usually prey on seals and sea lions but will, when this food source is scarce, seek out other prey.
B. There is no indication that substantial numbers of sea otters migrated to other locations from the North Pacific in the 1980's.
C. Along the Pacific coast of North America in the 1980's, sea otters were absent from many locations where they had been relatively common in former times.
D. Following the decline in the population of the sea otters, there was an increase in the population of sea urchins, which are sea otters' main food source.
E. The North Pacific populations of seals and sea lions cover a wider geographic area than does the population of sea otters.

Why A not E????

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Source: Beat The GMAT — Critical Reasoning | Tue Oct 04, 2011 7:31 am

mundasingh123: Legendary Member; Posts: 2330; Joined: Fri Jan 15, 2010 5:14 amFollowed by: 26 members; Thanked: 56 times

by mundasingh123 » Tue Oct 04, 2011 11:11 am

tracyyahoo wrote:In the late 1980's, the population of sea otters in the North Pacific Ocean began to decline. Of the two plausible explanations for the decline-increased predation by killer whales or disease-disease is the more likely. After all, a concurrent sharp decline in the populations of seals and sea lions was almost certainly caused by a pollution-related disease, which could have spread to sea otters, whereas the population of killer whales did not change noticeably.

Which of the following, if true, most seriously weakens the reasoning?

A. Killer whales in the North Pacific usually prey on seals and sea lions but will, when this food source is scarce, seek out other prey.
B. There is no indication that substantial numbers of sea otters migrated to other locations from the North Pacific in the 1980's.
C. Along the Pacific coast of North America in the 1980's, sea otters were absent from many locations where they had been relatively common in former times.
D. Following the decline in the population of the sea otters, there was an increase in the population of sea urchins, which are sea otters' main food source.
E. The North Pacific populations of seals and sea lions cover a wider geographic area than does the population of sea otters.

Why A not E????

Nice questions

I Seek Explanations Not Answers

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GmatKiss: Legendary Member; Posts: 2789; Joined: Tue Jul 26, 2011 12:19 amFollowed by: 43 members; Location: Chennai, India; Thanked: 206 times; GMAT Score:640

by GmatKiss » Tue Oct 04, 2011 11:29 am

Agree with Munda, Interesting question!
Am unable to figure out any weakening(most) option

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mundasingh123: Legendary Member; Posts: 2330; Joined: Fri Jan 15, 2010 5:14 amFollowed by: 26 members; Thanked: 56 times

by mundasingh123 » Tue Oct 04, 2011 11:36 am

Tracy i agree with the OA but have restrained from giving a reply because you have especially asked for an instructor

I Seek Explanations Not Answers

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GmatKiss: Legendary Member; Posts: 2789; Joined: Tue Jul 26, 2011 12:19 amFollowed by: 43 members; Location: Chennai, India; Thanked: 206 times; GMAT Score:640

by GmatKiss » Tue Oct 04, 2011 12:15 pm

mundasingh123 wrote:Tracy i agree with the OA but have restrained from giving a reply because you have especially asked for an instructor

Munda,

Its always good to pour in our thought, for it may be the point others are looking for.

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abhimanyu.tanwar: Senior | Next Rank: 100 Posts; Posts: 55; Joined: Wed Sep 22, 2010 9:09 am; Location: Pune, India; Thanked: 1 times

by abhimanyu.tanwar » Tue Oct 04, 2011 1:53 pm

tracyyahoo wrote:In the late 1980's, the population of sea otters in the North Pacific Ocean began to decline. Of the two plausible explanations for the decline-increased predation by killer whales or disease-disease is the more likely. After all, a concurrent sharp decline in the populations of seals and sea lions was almost certainly caused by a pollution-related disease, which could have spread to sea otters, whereas the population of killer whales did not change noticeably.

Which of the following, if true, most seriously weakens the reasoning?

A. Killer whales in the North Pacific usually prey on seals and sea lions but will, when this food source is scarce, seek out other prey.
B. There is no indication that substantial numbers of sea otters migrated to other locations from the North Pacific in the 1980's.
C. Along the Pacific coast of North America in the 1980's, sea otters were absent from many locations where they had been relatively common in former times.
D. Following the decline in the population of the sea otters, there was an increase in the population of sea urchins, which are sea otters' main food source.
E. The North Pacific populations of seals and sea lions cover a wider geographic area than does the population of sea otters.

Why A not E????

It has been explicitly stated in the passage that the population of seals and sea lions has declined sharply. the passage suggests that the reason for this decline is also disease. however, if we consider choice A which says "Killer whales in the North Pacific usually prey on seals and sea lions but will, when this food source is scarce, seek out other prey" we can infer that the killer whales must have first fed on seals and sea lions and when they (seals and sea otters) were less in number and hard to find killer whales shifted their focus on sea otters resulting in decline in sea otters' population. hence disease is not the reason for the decline. Weakens the reasoning.

Hope it helps!

Regards
Abhimanyu

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gunjan1208: Master | Next Rank: 500 Posts; Posts: 416; Joined: Thu Jul 28, 2011 12:48 amFollowed by: 6 members; Thanked: 28 times

by gunjan1208 » Tue Oct 04, 2011 9:46 pm

I think the question is interesting but easy also.
here below is my approach:-->

Before going to the choices, my contemplation about the stimulus:
1- For sure something that happened to other species can happen to otters as well but thats not always true.
2- Why can't whale eat otters. If I can put along a choice which supports this, objective achieved.

Thus I did zero in the choice A.

After that I rushed through other choices as I knew that A was the best one.

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sl750: Master | Next Rank: 500 Posts; Posts: 496; Joined: Tue Jun 07, 2011 5:34 amFollowed by: 1 member; Thanked: 38 times

by sl750 » Wed Oct 05, 2011 3:19 am

tracyyahoo wrote:In the late 1980's, the population of sea otters in the North Pacific Ocean began to decline. Of the two plausible explanations for the decline-increased predation by killer whales or disease-disease is the more likely. After all, a concurrent sharp decline in the populations of seals and sea lions was almost certainly caused by a pollution-related disease, which could have spread to sea otters, whereas the population of killer whales did not change noticeably.

Which of the following, if true, most seriously weakens the reasoning?

A. Killer whales in the North Pacific usually prey on seals and sea lions but will, when this food source is scarce, seek out other prey.
B. There is no indication that substantial numbers of sea otters migrated to other locations from the North Pacific in the 1980's.
C. Along the Pacific coast of North America in the 1980's, sea otters were absent from many locations where they had been relatively common in former times.
D. Following the decline in the population of the sea otters, there was an increase in the population of sea urchins, which are sea otters' main food source.
E. The North Pacific populations of seals and sea lions cover a wider geographic area than does the population of sea otters.

Why A not E????

The author of the argument suggests that between the two possible explanations for the diminishing number of the otters, the death of otters by pollution related diseases is the strongest. He goes on to support this claim by citing the sharp decline in sea lion population and suggests that the pollution could have spread to the otters too.

We are looking for,maybe, another reason for the decline in numbers of these species. This will weaken the authors claim that it was the pollution related diseases that caused the death of these creatures

Choice A provides just that evidence. This tells us that it was the killer whales that were responsible for the deaths of these sea creatures

Choice E doesn't help in weakening this claim

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Jim@Grockit: GMAT Instructor; Posts: 641; Joined: Wed Jul 22, 2009 3:07 pmFollowed by: 45 members; Location: Madison, WI; Thanked: 162 times; GMAT Score:760

by Jim@Grockit » Sun Oct 09, 2011 12:14 pm

Weakening causal arguments (pollution caused sea otter deaths) is done via three shortcuts:

1. Reverse the causal relationship (sea otter deaths caused pollution, not likely)
2. Deny the cause (here is some proof it wasn't pollution that caused the deaths)
3. Find another cause (it's those darn killer whales again, eating our sea otters!)

Choice A is a #3.

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mehrasa: Master | Next Rank: 500 Posts; Posts: 279; Joined: Fri Nov 05, 2010 5:43 pmFollowed by: 1 member; Thanked: 15 times

by mehrasa » Sun Oct 09, 2011 11:45 pm

IMO: A

in this argument two events occur at the same time, the author consider them cause-effect relationship...the argument conclude that the disease which killed sea lions is a cause for decline in population of otters...
since the argument claims that certainly the cause of decline in sea lion population is disease ==> their population declined ==> according to the answer choice A the killer whales find their main food source rare ==> go for another prey ==> hunt more otters ==> alternative cause for decline in otter population than disease ==> weaken the argument

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rohit_gmat: Master | Next Rank: 500 Posts; Posts: 158; Joined: Wed Jun 16, 2010 8:21 amFollowed by: 1 member; Thanked: 13 times

by rohit_gmat » Mon Oct 10, 2011 8:31 am

tracyyahoo wrote: Why A not E????

well the question asks us to weaken the conclusion.. the conclusion is that the otters have disappeared (died) due to diseases.
The question's premise states that there are ONLY two possibilities... one is death from disease and the other is due to predators..
so in order to weaken the conclusion we must try to proove that the deaths were caused by the predators.. only choice A does that

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