Summarize Argument
The author concludes that prehistoric rock paintings on small islands off the coast of Norway do not represent a description of the painters’ diets. This is because the author believes the painters needed to eat the sea animals in the waters around Norway in order to travel to the islands, and no paintings clearly show those sea animals.
Notable Assumptions
The author assumes that the paintings don’t depict sea creatures. (Just because they don’t unambiguously depict sea creatures doesn’t imply that they don’t show sea creatures.) The author also assumes that the painters needed to cross the waters to get to the islands. (Maybe the painters originated on the islands?) The author also assumes that the painters needed to eat sea creatures.
A
Once on these islands, the cave painters hunted and ate land animals.
This shows that the painters’ current diets did not need to include sea creatures. So the lack of sea creatures in paintings does not prove that the paintings don’t represent the painters’ current diets.
B
Parts of the cave paintings on the islands did not survive the centuries.
This raises the possibility that sea creatures might have appeared in paintings that are no longer around. So, the cave paintings could have been a description of the current diets of the painters, even if the paintings that remain no longer depict sea creatures.
C
The cave paintings that were discovered on the islands depicted many land animals.
The author never denied that the paintings could show land animals. The author’s position is that we’d expect to see sea creatures, too. So, the presence of land animals in the paintings doesn’t undermine the author’s argument.
D
Those who did the cave paintings that were discovered on the islands had unusually advanced techniques of preserving meats.
This raises the possibility that the painters did not need to eat sea creatures, contrary to the author’s assumption. The painters might have been able to travel to the islands while eating preserved meat from land animals.
E
The cave paintings on the islands were done by the original inhabitants of the islands who ate the meat of land animals.
This shows that the author’s assumption that the painters needed to cross the islands and eat sea creatures is wrong. The painters could have been done by inhabitants of the islands who may not have needed to eat sea creatures.
Summarize Argument
The statistician claims there’s reason to be skeptical about the magazine’s claim that North Americans are more concerned with finances than politics. Why? Because a question on the survey was biased and because subscribers to the magazine might poorly represent North Americans in general.
Notable Assumptions
The statistician assumes conclusions based on a survey with a self-selecting sample and a biased question should be received skeptically. He assumes the survey question he quotes is biased in a way that could affect the conclusion based on the survey’s results.
A
The credibility of the magazine has been called into question on a number of occasions.
This is another reason to view the survey’s results with skepticism. It suggests the magazine has been accused of being careless or insincere, which doesn’t prove the magazine’s conclusion wrong, but does give a reason to be skeptical of it.
B
The conclusions drawn in most magazine surveys have eventually been disproved.
This is another reason to view the magazine’s conclusion with skepticism. It suggests magazine surveys in general are unreliable, which doesn’t prove this magazine’s conclusion is false, but gives a reason to be skeptical about it.
C
Other surveys suggest that North Americans are just as concerned about politics as they are about finances.
This introduces new evidence that conflicts with the magazine’s conclusion, giving more reason to doubt that conclusion.
D
There is reason to be skeptical about the results of surveys that are biased and unrepresentative.
This makes concrete the statistician’s assumption that a survey with an unrepresentative sample and a biased question should be viewed skeptically.
E
Other surveys suggest that North Americans are concerned not only with politics and finances, but also with social issues.
This is irrelevant. Neither the magazine nor the statistician assumes politics and personal finance are the only two issues North Americans care about. Concern about a third issue gives no reason to doubt the magazine’s conclusion, which compares interest in the two issues only.
Multiple sclerosis is an autoimmune disease: white blood cells attack the myelin sheath that protects nerve fibers in the spinal cord and brain. Medical science now has a drug that can be used to successfully treat multiple sclerosis, but the path that led medical researchers to this drug was hardly straightforward. Initially, some scientists believed attacks characteristic of multiple sclerosis might be triggered by chronic viral infections. So in 1984 they began testing gamma interferon, one of the body’s own antiviral weapons. To their horror, all the multiple sclerosis patients tested became dramatically worse. The false step proved to be instructive however.
Summary
Scientists thought multiple sclerosis (MS) was caused by viral infections. To test this hypothesis, they used gamma interferon—a tool the body uses to combat viral infections—to treat MS. This experiment, however, resulted in the MS patients’ conditions worsening.
Notable Valid Inferences
Gamma interferon makes MS worse.
A
Gamma interferon stops white blood cells from producing myelin-destroying compounds.
This is not compatible with the experiment’s results. If gamma interferon does what (A) claims, it would interfere with the mechanism that makes MS so destructive. This would likely improve MS patients’ conditions—this contradicts the experiment, which shows the opposite effect.
B
Administering gamma interferon to those without multiple sclerosis causes an increase in the number of white blood cells.
This could be true and may explain why gamma interferon makes MS patients’ conditions worse. By causing an increase in white blood cells, gamma interferon could increase the number of cells attacking the myelin sheath in MS patients.
C
Medical researchers have discovered that the gamma interferon level in the cerebrospinal fluid skyrockets just before and during multiple sclerosis attacks.
This could be true and may explain why gamma interferon makes MS patients’ conditions worse. It suggests an association between increased gamma interferon levels and MS attacks.
D
It has now been established that most multiple sclerosis sufferers do not have chronic viral infections.
This could be true. It suggests that scientists’ initial belief that MS is triggered by viral infections is incorrect. This could explain why gamma interferon, one of the body’s antiviral tools, is ineffective against it.
E
The drug now used to treat multiple sclerosis is known to inhibit the activity of gamma interferon.
This could be true and may explain why gamma interferon makes MS patients’ conditions worse. It suggests that inhibiting gamma interferon levels improves patients’ conditions—this is consistent with the finding that increasing gamma interferon levels worsens patients’ conditions.
Summary
If monster is horrific, it’s threatening.
If monster is physically dangerous, it’s threatening.
If monster inspires revulsion, it’s horrific.
If monster is physically dangerous, it’s threatening.
If monster inspires revulsion, it’s horrific.
Notable Valid Inferences
If a monster inspires revulsion, then it’s threatening.
A
Any horror-story monster that is threatening is also horrific.
Could be false. We know that all monsters that are horrific are threatening. This doesn’t mean every monster that’s threatening is horrific. There can be threatening monsters that aren’t horrific (maybe they’re threatening for some other reason).
B
A monster that is psychologically dangerous, but that does not inspire revulsion, is not horrific.
Could be false. We know that monsters that inspire revulsion are horrific. That doesn’t imply monsters that don’t inspire revulsion are not horrific. They can still be horrific for other reasons besides revulsion.
C
If a monster triggers infantile fears but is not physically dangerous, then it is not horrific.
Could be false. We know that physically dangerous monsters are threatening. We also know that horrific monsters are threatening. This doesn’t imply that monsters that aren’t physically dangerous aren’t horrific. A monster can be horrific without being physically dangerous.
D
If a monster is both horrific and psychologically threatening, then it does not inspire revulsion.
Could be false. We know that monsters that inspire revulsion are horrific. This doesn’t imply that monsters that are horrific don’t inspire revulsion. It’s possible every monster that’s horrific and threatening inspires revulsion.
E
All monsters that are not physically dangerous, but that are psychologically dangerous and inspire revulsion, are threatening.
Must be true. The stimulus tells us that all monsters that inspire revulsion are horrific, and all monsters that are horrific are threatening. So, all monsters that inspire revulsion are threatening.
Summarize Argument
The author concludes that a hard track allows for faster running than a soft track, provided the conditions are dry. Why? Because on a hard track, runners spend less time with their feet touching the ground, even though their steps are shorter.
Notable Assumptions
The author assumes that runners save more time by keeping their feet off the ground than they lose by taking shorter steps on a hard track. In addition, he assumes there’s no other feature of hard tracks that allows people to run faster on them.
A
Dry running conditions can be guaranteed for indoor track races only.
This implies the conclusion isn’t always relevant, not that the argument is incomplete. The author restricts his statements to dry tracks only.
B
In general, taller runners have greater average step length than shorter runners do.
This doesn’t say shorter runners would benefit from a softer track, because it’s possible the shorter step length allowed by the harder surface holds back runners of all heights equally.
C
Hard tracks enhance a runner’s speed by making it easier for the runner to maintain a posture that minimizes wind resistance.
This calls the argument into question even though it gives another reason to support the conclusion. Perhaps hard tracks make for faster running because they reduce wind resistance, not because they allow for more air time.
D
The tracks at which the world’s fastest running times have been recorded are located well above sea level, where the air is relatively thin.
This suggests elevation is another factor that can affect running speed, but gives no difference between hard and soft tracks suggesting the reasoning is incomplete. The author does not claim that track surface is the only variable that might affect running speed.
E
To remain in top condition, a soft track surface requires different maintenance procedures than does a hard one.
This doesn’t say a soft track is any more or less likely to remain in top condition, nor what effect a track in poor condition would have on running speed.