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Re: In the late 1980’s, the population of sea otters in the North Pacific [#permalink]
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B - states that reason for decline of sea otters is not the migration to other location. But this doesn't weaken or streanthen the reason.

C - states that in 1980's population of sea otters decliend but no reason is stated for it.

D - states the affect of decline of sea otters. But this doesn't weaken or streanthen the reason.

E - Again just tells about population spread but nothing else which will weaken or streanthen the reason.

But A points out that possible reason for declie eof sea otters if whales started to kill them. So think A is the orrect choice.
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Re: In the late 1980’s, the population of sea otters in the North Pacific [#permalink]
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Conclusion - disease more likely the cause for population decline of sea otters
Premise- seals and sea lions population has come down but whale population is not affected.

Choice A clearly states that killer whales could be the cause of decline as they turn out to sea otters when there is a decline in seals and sea lions population.

Other choices IMO strengthens the arg.
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Re: In the late 1980’s, the population of sea otters in the North Pacific [#permalink]
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A for me as it points out that pollution was possibly not the reason for the decline in sea otters population. Thus weakening the reasoning.
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Re: In the late 1980’s, the population of sea otters in the North Pacific [#permalink]
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-- Conclusion is - diseases have caused sharp decline in sea otters. To weaken the argument, we need to find a reason other than the disease for sharp decline.

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.
-- Since killer whales had started preying on sea animals other than seals and sea lions, sea otters were the possible next option to them. Hence sea otters started declining in 1980's because killer whales started preying them. Hence A is correct answer choice.

B. There is no indication that substantial numbers of sea otters migrated to other
locations from the North Pacific in the 1980’s.
--Since there is no indication that sea otters migrated to location other than North Pacific, the possible cause of decline in not clear from this answer choice. Hence B cannot be a correct answer choice.

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.
-- Irrelevant. Hence cannot be a correct answer choice.

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.
--This answer choice mentions decline in sea otters, however fails to mention the possible cause for it. Hence it cannot weaken the argument and cannot be a correct answer choice.

E. The North Pacific populations of seals and sea lions cover a wider geographic
area than does the population of sea otters
-- Irrelevant. Hence E cannot be a correct answer choice.
Thank You.


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Re: In the late 1980’s, the population of sea otters in the North Pacific [#permalink]
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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

Let’s understand the details of the argument.
1) In the late 1980’s, the population of sea otters in the North Pacific Ocean began to decline.
2) Two possible explanations for the decline in the population of sea otters
• increased predation by killer whales
• disease

Author’s opinion/ Conclusion- Of the two explanations, disease is the more likely because-

There was a concurrent sharp decline in the populations of seals and sea lions caused by a pollution related disease, which could have spread to sea otters and there has been no noticeable change in the population of killer whales

We need to find an option that weakens the conclusion

(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.

This means that Killer whales preyed on sea otters because there was a decline in populations of seals and sea lions (usual prey). This weakens the conclusion that the decline in the population of sea otters is not because of disease but because of increased predation by killer whales. Hence A is correct.

(B) There is no indication that substantial numbers of sea otters migrated to other locations from the North Pacific in the 1980’s.

B brings in and eliminates another probable cause for the decline in the population of sea otters in the North Pacific. Eliminate B

(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.

The location of sea otters is not important. Eliminate C

(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.

Irrelevant. Eliminate D.

(E) The North Pacific populations of seals and sea lions cover a wider geographic area than does the population of sea otters

This is irrelevant to the argument. Eliminate E

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Re: In the late 1980s, the population of sea otters in the North Pacific [#permalink]
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Re: In the late 1980s, the population of sea otters in the North Pacific [#permalink]
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