Logical ReasoningDifficulty: Hard

PT133 S2 Q18 Explanation

The flagellum, which bacteria

A free, expert breakdown of this official LSAT Logical Reasoning question.

TopicsNecessary Assumption

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Stimulus

The flagellum, which bacteria use to swim, requires many parts before it can propel a bacterium at all. Therefore, an evolutionary ancestor of bacteria that had only a few no survival advantage from them.

What this question is testing

Necessary Assumption

Your task

Find the assumption the argument requires in order for its conclusion to hold.

Common trap

Answers that would help the argument but aren't strictly required (sufficient, not necessary).

Winning move

Negate each choice — the right one breaks the argument when negated.

Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.

The question
18.

Which one of the following is an assumption on which the

Answer choices

  1. Irrelevant Comparison8% picked this

    Any of bacteria's evolutionary ancestors that had only a few of the parts of the flagellum would be at a disadvantage relative to similar

    The comparison should be between bacteria with all of the parts of the flagellum and bacteria which do not have all of the parts.

  2. Correct60% picked this

    For parts now incorporated into the flagellum to have aided an organism's survival, they would have had

    Why this is right

    This bridges the gap between SA → SW providing a survival advantage and helping the bacteria swim.

    Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.

  3. Supports a Premise17% picked this

    All parts of the flagellum are vital to each of

    This supports the premise in SW → MP the argument.

  4. Out of Scope4% picked this

    No evolutionary ancestor of bacteria had only a few of the parts

    The conclusion is hypothetical, so it does not matter whether such an evolutionary ancestor existed.

  5. Negation12% picked this

    Any of bacteria's evolutionary ancestors that lacked a flagellum also lacked the

    This negates the premise of the F → SW argument, which says that bacteria use the flagellum to swim.

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