PrepTest 137

Incorrect. The conclusion specifically notes "even if . . . black water . . . has struck the bay before," so it's not assuming that it has, in fact, done so.

Incorrect. The argument notes that 5 species were wiped out, but it doesn't discuss any other species. It's possible that some others are completely immune to black water, so this answer is too extreme in stating something is true of "every" species in the bay.

Incorrect. If anything, this cuts against the argument by suggesting that the black water wasn't all that damaging.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Last year, black water knocked out 5 species of coral, some of it over 200 years old. Therefore, black water of last year's intensity hasn't hit in the past 200 years.

Answer Anticipation:
This argument is interesting in that it introduces a viewpoint that it attributes to "[s]ome scientists," but it never really pivots away from it. The author's viewpoint isn't 100% in agreement with theirs, but they're in the same world.

What is the author's conclusion? That any black water from the past 200 years wasn't at the same intensity as last year's blackwater. The reason for this is that last year's black water killed some coral that was over 200 years old. In other words, the explanation the author of this argument provides as to why the coral died in this year's black water was the intensity of that black water.

Whenever the LSAT provides an explanation in the conclusion, your first thought should be to alternatives. Is there a way that black water of completely normal intensity could have killed coral that withstood it before? Sure—maybe the coral was weaker. Or maybe something else was present with the black water that made the coral particularly susceptible to the black water. Maybe older coral is more vulnerable to black water.

Since this Strengthen with Necessary Premise question concludes that there is a specific explanation for a given phenomenon, it's assuming that no other explanation is correct. As such, the correct answer here will likely eliminate one of these possibilities. Let's stay flexible and look for any answer that brings up a way for black water to kill coral without it being particularly intense.

Answer Explanation:
This answer rules out a potential alternative explanation for the 200 year old coral dying. It's possible that it wasn't the intensity of the black water that killed the coral, but that something had weakened that coral before the black water came in. The argument needs to rule that possibility out for it to work, so this is our answer.

Key Takeaway:
In Strengthen with Necessary Premise questions, when an argument concludes one explanation for a phenomenon, the correct answer will likely rule out an alternative explanation. In fact, in this question type in general, correct answers will often rule out alternatives (e.g., alternative causes).

Incorrect. Two issues here. If anything, in raising the possibility that old coral is vulnerable, this answer establishes an alternative explanation for why that 200 year old coral was wiped out, which would weaken the argument, making this answer incorrect. Second, this answer only establishes relative vulnerability—this answer doesn't actually establish the old coral was particularly vulnerable, or more vulnerable than it had been earlier in its lifetime.