PrepTest 107

[lcid:3527] Prep Test 107 LSAT — Logical Reasoning — S4 Logical reasoning

Question prompt

Quasars—celestial objects so far Remaining source text redacted.
Why the credited answer is right

Credited answer: E

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

Quasars - Light takes 500m+ years to reach Earth; seen since 1963
Appear on Earth like quasars from that far → Burn steady with fire of 90b+ suns
Burn steady with fire of 90b+ suns → ~Exist for more than 100m years

Answer Anticipation

There’s a lot of science in the stimulus here, but ignore the topical language and focus on the logical language. We learn something about quasars, and then we get some general rules that we can express as conditionals.
The second sentence talks about anything that appears as a quasar to Earth - it must burn at a certain rate. And then the next statement tells us that anything burning at that rate can’t exist for more than a certain amount of time.
We can chain those two together, leading to the inference that anything that looks to Earth like a quasar can’t exist for more than 100m years. Putting that together with the first statement, we know that the light from the quasar takes over 500m years to reach Earth. Since a quasar can only exist for, at most, 100m years, by the time its light gets to Earth, the quasar must no longer exist. Let’s find an answer stating that.

Answer choices

  1. A
    Instruments in use before Remaining source text redacted.
    Why choice A is not credited
    This answer provides one reason that quasars weren’t seen until 1963, but it’s not the only. Maybe quasars didn’t exist before a certain time that meant their light first reached Earth in 1963, or maybe instruments were sensitive enough to see quasars as long as they were looking for them, and people weren’t. Since this answer is just one potential explanation for a phenomenon in the stimulus, it doesn’t have to be true.
  2. B
    Light from quasars first Remaining source text redacted.
    Why choice B is not credited
    That’s one explanation for why quasars have been seen since 1963, but it could be that we didn’t have instruments that could see quasars until then!
  3. C
    Anything that from Earth Remaining source text redacted.
    Why choice C is not credited
    This answer is close to repeating the first conditional. If it did, it would be correct. However, that conditional talks about objects that appear bright like a quasar from over 500m light years away, not anything that appears that bright. After all, the sun is the brightest thing we can see from Earth, and it burns with the light of 1 sun.
  4. D
    Nothing that is as Remaining source text redacted.
    Why choice D is not credited
    Nothing that is as far from Earth as quasars are and appear as bright as they do can exist for more than 100m years, but there may be things that far away that don’t burn that brightly that can last longer.
  5. E
    No quasar that has Remaining source text redacted.
    Why choice E matches the stem
    The conditionals allow us to infer that quasars don’t exist for more than 100m years, and yet it takes over 500m years for the light to reach Earth. Therefore, by the time quasar light reaches Earth (500m years), the quasar has burned out (after 100m years). This answer is therefore correct.

What this tests

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