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- Posts: 8950
- Joined: Feb 02, 2011
- Fri Jan 21, 2011 12:00 am
#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:
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 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:
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:
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:
advantage, so this answer choice is incorrect.
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.
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.
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.”
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.”
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.”
advantage, so this answer choice is incorrect.
