Critical Reasoning

In the late 1980s, the population of sea otters in the North Pacific began to decline. There are two plausible explanations for the decline: predation, possibly by killer whales, or disease. Of these two, disease is the more likely, since a concurrent sharp decline in population of seals and sea lions is believed to have been caused by disease, and diseases that infect these creatures are likely to be able to infect sea otters also.
Which of the following, if trure, 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 the sea otter population at any North Pacific
location declined in the 1980s because of substantial numbers of sea otters
migrating to other locations.
C.Along the Pacific coast of North America in the 1980s, 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 seal and sea lions cover a wider geographic
area than does the population of sea otters.

A looks the best choice to me.

The argument says that disease is more likely a cause because the disease has affected seals and sea lions and thus, ruling out the possibility of predation by killer whales without any reason. A counters by saying that it is not because of the disease but because of predation.

the author mentions 2 possible explanations for the decline- predation and disease.
He outrightly eliminates the predation component without justifying it. so in order to weaken the stimulus we need to touch upon this concept.

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. [ gives another explanation for the decrease in population other than that caused by disease. Correct!]

B.There is no indication that the sea otter population at any North Pacific location declined in the 1980s because of substantial numbers of sea otters migrating to other locations. [it is totally unrelated to the topic at hand. we are just trying to say other possibilities have been ruled out. it still does not bring clarity to why predation as cause can be ignored. Eliminated.]

C.Along the Pacific coast of North America in the 1980s, sea otters were absent from many locations where they had been relatively common in former times. [this just gives us info that the population of sea otters has declined. does not affect the conclusion in any form. Eliminated!]

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 is totally unrelated. what happens after the decline of sea otters is not our concern. we concerned about providing another reason for the decline in the population. Eliminated!]

E.The North Pacific populations of seal and sea lions cover a wider geographic area than does the population of sea otters. [ does this weakon any statement at all? this does not address the issue at hand. Eliminated!]

Hope this helps!

In the late 1980s, the population of sea otters in the North Pacific began to decline. There are two plausible explanations for the decline: predation, possibly by killer whales, or disease. Of these two, disease is the more likely, since a concurrent sharp decline in population of seals and sea lions is believed to have been caused by disease, and diseases that infect these creatures are likely to be able to infect sea otters also.
Which of the following, if trure, 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 the sea otter population at any North Pacific
location declined in the 1980s because of substantial numbers of sea otters
migrating to other locations.
C.Along the Pacific coast of North America in the 1980s, 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 seal and sea lions cover a wider geographic
area than does the population of sea otters.



Conclusion: The sea otters have declined in numbers because of the disease and not because of predation by killer whales. (As these are only two plausible explanations)

Premise: 1. sharp decline in population of seals and sea lions is believed to have been caused by disease
2. diseases that infect these creatures are likely to be able to infect sea otters also.
3. There are two plausible explanations for the decline: predation, possibly by killer whales, or disease.
4. In the late 1980s, the population of sea otters in the North Pacific began to decline.

Can we find the assumption or prephase the answer which weakens?

Note that it is not sure that seals and sea lions have been actually infected (is believed) by disease and so are the seal otters.


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.

If this is true, then it can explain both the delcine of seals and sea lions as well as the sea otters. but dont conclude unless and until all options are gone through. We want the choice that most seriously weakens. Seriously it does, is it most? Lesse.


B.There is no indication that the sea otter population at any North Pacific
location declined in the 1980s because of substantial numbers of sea otters
migrating to other locations.

Not a plausible explanation for decline, so not to be considered. And it does not affect the conclusion that they declined because of disease and not because of killer whales.


C.Along the Pacific coast of North America in the 1980s, sea otters were
absent from many locations where they had been relatively common in
former times.

Dont get bogged down by alternative explanation. kEEP focus of Conclusion in the mind.
Also it does not give any alternate explanation. It just says that they were not found in 1980S. Does not have any effect on conclusion.

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.

What happened next, does not affect conclusion.

E.The North Pacific populations of seal and sea lions cover a wider geographic
area than does the population of sea otters.

How does this affect conclusion?

A is our most serious answer. Not much efforts required in eliminating the rest if conclusion is transfixed in mind.

Conclusion: Of the two reasons (predation and disease), disease is more likely to be the cause of the decline in the population.

premises leading to the conclusion,

concurrent decline in the populations of seals and sea-lions because of the disease(believed).
Diseases are equally infective to the sea otters.

Now to weaken this argument we just need to prove that the decline in numbers might not be only because of the disease.

This is what A does.

I think A is the right answer here.