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Complete Question Explanation

Assumption. The correct answer choice is (B)

In this stimulus the author presents a basic argument, as follows:
  • Premise: ..... Only certain plant species’ phytoliths have been found on the teeth of a particular herbivorous (plant-eating) extinct great ape.

    Conclusion: ..... The ape’s diet must have consisted only of those species.
The author must presume that if a plant didn’t leave phytoliths, then the ape didn’t eat that plant:
  • ..... ..... ..... ..... phytoliths ..... :arrow: ..... part of diet

    contrapositive: ..... part of diet ..... :arrow: ..... left phytoliths
Answer choice (B): This is the correct answer choice. The answer to this supporter assumption question should reflect either the conditional statement above, or its contrapositive. Answer choice (B) provides the necessary link, which can be diagrammed as follows:
  • part of diet ..... :arrow: ..... left phytoliths
Answer choice (A) is incorrect, since the stimulus does not deal with the continued existence of the plants consumed, but with the question of which species were part of the diet of the great ape under discussion. Incorrect answer choice (C) deals with consistency from tooth to tooth, which is not relevant to the discussion of whether all plant species consumed left phytoliths on the teeth of the extinct great ape. Answer choices (D) and (E) are both incorrect for the same reason: they each deal with a species other than the one dealt with in the stimulus (which is one particular species of herbivorous great ape).
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Although I got the right answer, I found the stimulus to be confusing. The student center states that the author assumes this conditional statement: "no phytoliths --> not part of diet." How did the author indicate this assumption (although it does make sense). But in a purely mechanical sense (with sufficient/necessary indicators), the conclusion seems to have two necessary indicators: "Since only phytoliths from certain species of plants are found on the teeth, the apes' diet must have consisted only of those plants."

Please help :)
 Robert Carroll
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"Since" indicates a premise. Part of that sentence is evidence for the conclusion at the end. Thus:

found on teeth :arrow: certain plant species

And the conclusion:

apes' diet :arrow: certain plant species

This conclusion would follow if we could add the conditional:

apes' diet :arrow: found on teeth

And indeed, answer choice (B) says that.

An assumption is not something the author makes explicitly. Thus, there is no place to look in the stimulus for it. It's that the author's argument makes no sense unless the author is tacitly assuming the assumption we're looking for. The conclusion does not follow from the premise. The author didn't make a bad argument for no good reason. Thus, the author assumed the connection in answer choice (B), but did not explicitly state it.

Robert Carroll
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I dont understand how the answer for this cannot be C. The explanation of why it isnt C doesnt click in my head. The passage states " since ONLY phytoliths from certain species of plants are found on the teeth, the apes' diet MUST have consisted ONLY of those plants." C states that each toothed examined had phytoliths of the SAME plant species, this connects with saying that ONLY phytoliths from certain species of plants are found on the teeth. Answer B says plants of every type left phytoliths but it says ONLY phytoliths from certain species...

I dont understand

Thank you!

Lexi Gibbs
 Shannon Parker
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Hi Lexi,

Answer Choice (C) only states that the phytoliths were found on every tooth examined, it does not address the existence of other plants in the herbivore's diet. As the explanation above demonstrates, the only answer choice that addresses the complete diet of the herbivore, "every type eaten." By providing the link between the whole diet and phytoliths left on the teeth, it supports the conclusion.

Hope this helps clear it up some.

 Adam Tyson
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I'll add my two cents here on what's wrong with answer C, and it's the difference between "that's all we found" and "we found all of that everywhere."

Let's forget about phytoliths and ancient apes - let's make this more personal. Let's talk about college freshmen. First time away from home, mom not there to cook them a good dinner, the cafeteria isn't cool, so what do they do? They live on mostly pizza and ramen noodles and snickers and monster. Mostly, but not exclusively.

Now, you're a scientist, and you uncover the remains of some college freshmen, and you study their teeth to find out what they ate. You find pizza, and snickers, and ramen, and monster, and nothing else, so you conclude that that's all they ever ate. What's wrong with this conclusion? It ignores the possibility that they brushed their teeth and removed all traces of something - maybe they had mac and cheese at the cafeteria the day before they died, or maybe they went home and had some green beans and baked chicken with Mom and Dad the prior weekend. I suppose it's possible they drank some beer, but probably not, since they were freshman and underage and of course they would not do that. But maybe. Maybe they ate something, maybe some apples, that miraculously left no trace in their teeth - it all went down the hatch without leaving any evidence that it was ever there.

So there is the problem - just because all we found was the junk food doesn't prove that all they ate was the junk food.

So what about C? Does it matter that they found pizza on some teeth, and snickers on other teeth, and ramen on some other teeth, but not every tooth had each thing on it? Pizza on the molar, snickers on the bicuspid, monster on the tonsils, etc. - is that a problem? Not at all - they never said that the food they ate left exactly the same stuff on each and every tooth. It's about the cumulative findings - we found a total of four foods on the teeth, not necessarily four things on each tooth.

Chew on that explanation a while and see if you can digest it!

Now I feel like I need to brush my teeth. Or eat a snickers. See ya!

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