PrepTest 105

[lcid:3517] Prep Test 105 LSAT — Logical Reasoning — S2 Logical reasoning

Question prompt

Brown dwarfs—dim red stars Remaining source text redacted.
Why the credited answer is right

Credited answer: A

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

Argument or Facts

Argument

Valid or Flawed

Flawed

Question Type

Strengthen with Necessary Premise Questions

Stimulus Summary

Brown dwarfs - Can’t burn hydrogen Red dwarfs - Can burn hydrogen Brown and red dwarfs look similar Star → A lot of lithium when formed Star AND ~Coolest brown dwarf → Hot enough to turn all lithium into helium Therefore - Star AND ~Lithium → ~Coolest brown dwarf

Answer Anticipation

Wow, this is a doozy. This is a notoriously hard question, and the science definitely makes it harder, but let’s dig in. The argument starts by bringing up some information about brown and red dwarfs. However, the conclusion ends up being just about brown dwarfs, so we can probably ignore that info about red dwarfs - and their similarity to brown dwarfs. It’s probably there to throw us off! Same with hydrogen - the conclusion is about lithium (and helium, since they’re connected). Remember - the conclusion sets the scope of the argument. And in the science passages, they’re going to try to get you to confuse terms and think things are important that aren’t don’t fall for it. From there, we get information about stars generally. First, they all start out with lithium. Secod, outside of the “coolest” brown dwarfs, they all burn hot enough to turn all lithium into helium. So any star except the coolest brown dwarfs (not even all brown dwarfs) will convert all their lithium to helium. From this, the argument concludes that a star with no lithium in it won’t be one of the coolest brown dwarfs. Or, in other words, all of the coolest brown dwarf stars contain lithium. So where’s the flaw in that? Well, we know that the coolest brown dwarf stars aren’t hot enough to burn off the lithium, and they started out with lithium. However, that doesn’t mean that they still have lithium - maybe they started off hotter, burned off the lithium, and then cooled down. The argument sets up that stars change over time by stating that they are all “first” formed with lithium but then burn it off, so it’s possible that the brown dwarfs cool down over time. The argument assumes that this didn’t happen - that the brown dwarfs didn’t burn off their lithium before cooling down, so let’s find an answer establishing that.

Answer choices

  1. A
    None of the coolest Remaining source text redacted.
    Why choice A matches the stem
    Right off the bat, we get an answer that matches our anticipation. If some of the coolest brown dwarf stars were, at some point, hot enough to destroy lithium, then there might be very cool brown dwarf stars out there without lithium in them. That would completely undermine the conclusion, so this is a necessary premise.
  2. B
    Most stars that are Remaining source text redacted.
    Why choice B is not credited
    We know that brown dwarfs are too cool to burn hydrogen, but the conclusion is about a subset of these stars - the coolest of them. The coolest likely don’t make up over 50% of all brown dwarf stars - you wouldn’t say that the person who was taller than just over half the class is among the shortest! - so this is too strong to be a necessary premise.
  3. C
    Brown dwarfs that are Remaining source text redacted.
    Why choice C is not credited
    Destroying helium is out of scope - we know that stars turn lithium into helium, but there’s no discussion of what happens to the helium after that.
  4. D
    Most stars, when first Remaining source text redacted.
    Why choice D is not credited
    The argument establishes that all but the coolest brown dwarf stars burn hot enough to destroy lithium completely, so it doesn’t seem to matter to them how much lithium they start out with!
  5. E
    No stars are more Remaining source text redacted.
    Why choice E is not credited
    The similarity in appearance between any stars, let alone brown and red dwarfs and other stars, is unmentioned and out of scope of the conclusion about what elements are present in the stars, so this answer is out of scope.

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