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#36515
Complete Question Explanation

Assumption. The correct answer choice is (B)

This stimulus discusses the flagellum, which is used by bacteria to swim and requires many
parts before it can propel the bacteria at all. Based on this premise, the writer concludes that an
evolutionary ancestor of bacteria that had only a few flagellum parts gained no survival advantage
from those parts:
  • Premise: The flagellum requires many parts in order to propel a bacterium.

    Conclusion: A bacterial ancestor that was missing some flagellum parts would gain no
    survival advantage from the flagellum.
We should note that there is a significant logical leap represented here, from the premise, which
concerns an inability to aid in swimming, to a conclusion about an inability to provide any survival
advantage.

The question stem asks us to identify the assumption on which the stimulus’ conclusion depends.
This is a Supporter Assumption question, and the correct answer choice should link together the two
elements discussed above to allow the author to properly draw the conclusion that an incomplete
flagellum would offer no advantages if it were unable to propel a bacterium.

Answer choice (A): If an incomplete flagellum actually served as a disadvantage, this would justify
the conclusion in the stimulus, but it is not an assumption required by the argument. That is, the
assertion in this answer choice goes beyond what is necessary for the argument to stand—we don’t
need to know that such ancestors would be at a disadvantage—only that there would be no survival
advantage associated with limited parts of the flagellum. Since the assumption provided by this
answer choice is not required by the argument, this answer choice is incorrect.

Answer choice (B): This is the correct answer choice. This answer choice provides the required
link between the survival advantage, the parts of the flagellum and the ability to swim:
  • Premise: The flagellum requires many parts in order to propel a bacterium.

    Assumption: For the flagellum to offer any survival advantage, it would have to aid in the
    ability to swim (that is, it would have to be able to propel the bacterium).


    Conclusion: Therefore, a bacterial ancestor that was missing some flagellum parts would
    gain no survival advantage from the flagellum.
The above assumption is required to allow the author’s conclusion to be logically drawn from the
premise provided in the stimulus.

Answer choice (C): While this assertion might lend support to the conclusion in the stimulus, it is
not required by the argument, so this answer choice cannot be correct.

To check our work, we can apply the Assumption Negation Technique to determine whether the
negated assumption would weaken the conclusion:
  • “Not all parts of the flagellum are vital to each of its functions.”
Even if some of the flagellum's parts don’t play a role in every flagellum function, this doesn’t
weaken the argument asserting the need for a full flagellum to derive any survival benefits.

Answer choice (D): The fact that no evolutionary ancestor of bacteria had been limited to a few
parts of the flagellum—if this were the case—would not play into the argument, which is phrased
in the conditional tense, asserting that if there were such an ancestor, no survival advantage would
be gained. The assertion in this answer choice is not an assumption required by the argument in the
stimulus.

Again, we can apply the assumption negation technique by determining the effects of the negated
version of the answer choice:
  • “Some evolutionary ancestors of bacteria had only a few of the parts of the flagellum.”
Since this assertion would not weaken the argument in the stimulus, it is confirmed that this answer
choice does not supply an assumption on which the conclusion relies.

Answer choice (E): This answer choice reinforces a point already covered by the stimulus—that
bacteria use their flagella to swim. This is not, however, an assumption required by the argument,
and since there is no mention of survival advantage, this cannot be the supporter assumption that ties
together the author’s argument.

To check our work, we can apply the assumption negation technique and determine whether the
argument in the stimulus is weakened by the negated version of the answer choice:
  • “Not all of the bacteria’s flagellum-lacking evolutionary ancestors also lacked the capacity to
    swim.”
Again, this assertion has no effect on the argument that an incomplete flagellum provides no survival
advantage, so this answer choice is incorrect.
 catherinedf
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#3374
Thanks! That's really helpful. I totally forgot about using the negation technique - when you use it, it makes a whole lot more sense.

Thanks again!

Catherine
 LSAT2018
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#45341
Would I be able to use conditional reasoning to solve this problem?

Swim → Many Parts
Survival Advantage → Many Parts
Survival Advantage → Swim
 James Finch
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#45359
Hi LSAT 2018,

I don't think conditionalizing the stimulus would help. With assumption questions, you're looking for either a new element introduced into the stimulus's conclusion (a Supporter assumption) or else an outside line of attack that would invalidate the argument if not true (Defender assumption). Here we see that the conclusion introduces the idea of a "survival advantage," which it links to not having many parts of the flagellum. However, all we know is that not having many parts means that bacteria can't swim, not that they can't have a survival advantage. So based on this, we can prephrase that the correct answer choice will link a survival advantage with being able to swim. Answer choice (B) does this, and is the correct answer, which we can test using the Assumption Negation technique described in the original post.

Hope this helps!
 student987
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#49700
Hello! I went to the correct answer on this one, but did so hesitantly because of the word "now" in the answer choice (B). I thought that because the conclusion is talking about an ancestor of the bacteria, the assumption would involve bacteria of the past. Could you please explain how the ancient/current time concepts are at play here?
 Adam Tyson
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#49987
The premises ARE about "now," student987! "Which bacteria use to swim" is a present-tense statement - right now, that is what bacteria use the flagellum for. "These parts" also refers to the present - it's referring to the many parts that are currently required to propel a bacterium. If a flagellum has only have a few, rather than the many that are required, then at least today it couldn't swim.

What was your prephrase here? Did you know to look for a link between swimming and survival? Something like "if it didn't allow them to swim, it didn't help them survive"? Having just that much should be enough to eliminate all the losers here, since none of them link those two concepts.
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 smtq123
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#93622
Hi,

I would appreciate if anyone can provide the negation of option B and explain how the negation destroys the conclusion.

My negation of option B is: For parts now incorporated into the flagellum to have aided an organism’s survival, they would NOT have had to help it swim.

In my view swimming is not as important point as it is mentioned as a non-essential modifier in the stimulus. So, even if the added parts are unable to make it swim, they are still helping it to survive.

Kindly clarify.

Many thanks in advance!
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 Beth Hayden
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#93778
Hi SMTQ,

This is a tricky one to negate, but I would say this:

It is possible for parts now incorporated into the flagellum to have aided an organism's survival that did not help it swim. In other words, maybe even though you need a bunch of parts to be able to swim, you could still have some survival advantages from just a few.

If that's true the argument doesn't make sense. The argument reasons that if an ancestor only had a few parts of the flagellum, it would not have been able to swim, so that must mean that it would have no survival advantage. But if it's possible for it to have gotten survival benefits even though there weren't enough parts for it to be able to swim, the argument fails.

Hope that helps!
Beth
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 goingslow
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#95277
Hi there! I have a question about (C) - is it a Sufficient Assumption?

(C) says "All parts of the flagellum are vital to each of its functions". I understand it as saying "All parts of the flagellum are necessary to each of its functions". Is that correct?

If so, having only some parts of the flagellum means NO functions of the flagellum. Would that guarantee the stimulus's conclusion that "only a few of these parts would gain no survival advantage from them"? Or is "no functions" not the equivalent of "no survival advantage"?

Any thoughts would be appreciated. Thank you so much!
 Robert Carroll
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#95321
goingslow,

I think I agree with you, answer choice (C) might prove the conclusion, but it makes irrelevant the discussion in the stimulus about swimming, so it's too strong for an Assumption question. Might be good for a Justify, though!

Robert Carroll

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