PrepTest 106

[lcid:3520] Prep Test 106 LSAT — Logical Reasoning — S1 Logical reasoning

Question prompt

The axis of Earth's Remaining source text redacted.
Why the credited answer is right

Credited answer: B

The notes below walk through why it fits the stem and how to eliminate the rest.

Argument or Facts

Facts

Valid or Flawed

NA

Question Type

Must Be True Questions

Stimulus Summary

Earth’s angle stable → Gravity of Moon
~Earth’s angle stable → ~Life

Answer Anticipation

Note that we cut the first and last statement from our summary. Why is that? Because they’re not the kinds of statements that lead to inferences! The opening line is a fact about the Earth’s axis, and it’s hard to see what a correct answer could infer from that without just directly restating it. The last line is about a single example - again, what inference can be drawn from that other than just a restatement?
Instead, let’s focus on the second and third statements, both of which are conditionals.
First, how did we diagram them? The first conditional has a traditional keyword - “only.” That introduces a necessary condition, so the gravity of the Moon is necessary for the stability of Earth’s axis. The second conditional uses “Without” as an indicator word, which we treated as “if not,” introducing a negated sufficient condition. We also shortened that one a bit to just be about there being no life on the planet, but we can always go back and check for more details if an answer generally lines up with our anticipation.
With those two noted, we can see that we’ll need to take the contrapositive of one to make our chain. Let’s take the contrapositive of the second, and then contrapose the whole chain to have all potential answers:
Life → Earth’s angle stable → Gravity of Moon
~Gravity of Moon → ~Earth’s angle stable → ~Life

Answer choices

  1. A
    If Mars had a Remaining source text redacted.
    Why choice A is not credited
    The gravity of the Moon is noted as being necessary for life on Earth, not sufficient for it. Thus, even if Mars had a similar moon, there’s no guarantee it could support life.
  2. B
    If Earth's Moon were Remaining source text redacted.
    Why choice B matches the stem
    Looking at the contrapositive, we can infer that if the Moon leaves (taking its gravity with it), then the Earth’s axis wouldn’t be stable, and life wouldn’t be possible. This answer is thus supported by the stimulus, so it’s the correct answer.
  3. C
    Any planet with a Remaining source text redacted.
    Why choice C is not credited
    First, this answer reverses the logic of the second conditional premise, where the stability is necessary for life, not sufficient for it. Second, there’s no indication that what is true of Earth’s axis and its relationship to life on Earth is true of every other planet.
  4. D
    Gravitational influences other than Remaining source text redacted.
    Why choice D is not credited
    The stimulus doesn’t discuss the gravitational influence of any other source, so this answer is out of scope. For example, other sources might determine that the Earth’s axis is at a 23 degree angle, and the Moon makes that angle stable.
  5. E
    No planet that has Remaining source text redacted.
    Why choice E is not credited
    Mars has more than one moon and can’t support life, but we can’t generalize from that one example. And none of the conditionals support this answer, so we can eliminate it.

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