GMATPrep - The milk of many mammals

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GMATPrep - The milk of many mammals

by TheGraduate » Wed Jul 05, 2017 5:52 am
The milk of many mammals contains cannabinoids, substances that are known to stimulate certain receptors in the brain. To investigate the function of cannabinoids, researchers injected newborn mice with a chemical that is known to block cannabinoids from reaching their receptors in the brain. The injected mice showed far less interest in feeding than normal newborn mice do. Therefore, cannabinoids probably function to stimulate the appetite.

Which of the following is an assumption on which the argument depends?

A. Newborn mice do not normally ingest any substance other than their mothers' milk.
B. Cannabinoids are the only substances in mammals' milk that stimulate the appetite.
C. The mothers of newborn mice do not normally make any effort to encourage their babies to feed.
D. The milk of mammals would be less nutritious if it did not contain cannabinoids.
E. The chemical that blocks cannabinoids from stimulating their brain receptors does not independently inhibit the appetite.

Could anyone please explain why E is better than B?

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by DavidG@VeritasPrep » Wed Jul 05, 2017 6:01 am
TheGraduate wrote:The milk of many mammals contains cannabinoids, substances that are known to stimulate certain receptors in the brain. To investigate the function of cannabinoids, researchers injected newborn mice with a chemical that is known to block cannabinoids from reaching their receptors in the brain. The injected mice showed far less interest in feeding than normal newborn mice do. Therefore, cannabinoids probably function to stimulate the appetite.

Which of the following is an assumption on which the argument depends?

A. Newborn mice do not normally ingest any substance other than their mothers’ milk.
B. Cannabinoids are the only substances in mammals’ milk that stimulate the appetite.
C. The mothers of newborn mice do not normally make any effort to encourage their babies to feed.
D. The milk of mammals would be less nutritious if it did not contain cannabinoids.
E. The chemical that blocks cannabinoids from stimulating their brain receptors does not independently inhibit the appetite.

Could anyone please explain why E is better than B?
Conclusion: cannabinoids stimulate the appetite
Premise: when injected with a chemical that blocked cannabinoids, the mice lost their appetites

Try negating both B and E. The correct answer, when negated, will undermine the argument.

B negated: Cannabinoids are NOT the only substances in mammal's milk that stimulate the appetite.
This doesn't really impact the conclusion. Even if cannabinoids are one of many substances that stimulate the appetite, it would still make sense that if cannabinoids do make mice hungry, they'd become less hungry when the cannabinoids were blocked. (And remember, we're told that the mice lose their appetite. That part isn't in doubt. The question is whether it's the blocking of the cannabinoids that's responsible.)

E negated: The chemical that blocks cannabinoids from stimulating their brain receptors DOES independently inhibit the appetite. If this is true, then suddenly we don't know if it was the blocking of the cannabinoids that suppressed the appetite of these mice, or if it was the chemical itself that was responsible, independent of its function on cannabinoids. Because this answer choice undermines the argument, E is correct.
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