Q11 - A mass of "black water"

[ohthatpatrick]

Awesome work.

What does the Question Stem tell us?
Necessary Assumption

Break down the Stimulus:
Conclusion: last year's black water was more intense than black water episodes of the past two centuries.
Evidence: last year's black water mass wiped out coral that were more than two centuries old. (i.e., the previous black water episodes clearly did NOT wipe out this coral, since the coral is more than two centuries old)

Any prephrase?
If you wanted to argue with this guy, you might say: Why do we have to assume this recent black water was super strong? It could be that every black water episode is similar intensity, and with each new episode the coral gets more and more weakened. This last black water episode was just the straw that broke the coral's back -- sorry, couldn't resist.

Answer choice analysis:
A) is not something the author needs to assume because it's focused on the frequency of black water occurrences. The author's conclusion is focused on the INTENSITY of previous black water occurrences. The disclaimer phrase "even if this black water phenomenon has struck the bay before" leaves the author open to believing that it has happened before / hasn't happened before / has happened frequently / hasn't happened frequently.

B) is wrong because of the extreme word "EVERY". The author's argument is based on the damage done to the 5 species of coral he mentioned. Nothing needs to be assumed about EVERY species of coral.

C) is wrong because it deals with the secondary effects of the black water that go beyond the scope of the evidence/conclusion. If you negate it, (C) says that the black water from last year DID decimate other plants/animals that rely on coral. So what? That doesn't hurt the argument in any way.

D) I think you were wise to negate (D). I find that answers that are phrased negatively "it is NOT the case that ...", "the mounds were NOT especially fragile ..." are the most fruitful times to apply the negation test. If we negate D, it badly weakens the argument. It offers an alternative explanation for why the mounds of coral were wiped out (it wasn't an especially bad black water episode; the mounds were just weak)

E) does not need to be assumed because nothing in the premise/conclusion suggested the author was concerned with making a distinction between older coral and younger coral. I tend to call these common trap answers "fake comparisons". Whether older or younger coral are more susceptible to black water has no bearing on us analyzing the issue of the conclusion: whether last year's black water was or wasn't more intense than previous episodes.

The correct answer is D.

Takeaway/Pattern: About 1/2 of the correct answers to Necessary Assumption are really ruling out a potential objection, alternative interpretation of evidence, or potential obstacle that would thwart a prediction from coming true.

#officialexplanation

Keep up the good work.

[Shiggins]

This question at first seemed confusing but I believe I was able to come to grips with it.

It says the black water phenomenon came in last year and caused damage.

This phenomenon is naturally occurring and is infrequent.

The author concludes even if it had struck the day before it would not of been as intense

So at first the conclusion threw me off, but I took it as the phenomenon was stronger last year it hit than it had been in the past two centuries.

I thought what evidence does he have. Well the damage from the last black mass. But what if that was a fluke. Choice D states that. If you negate D, then the coral was already weak to begin with, thus ruining it being intense.

[wgutx08]

About 1/2 of the correct answers to Necessary Assumption are really ruling out a potential objection, alternative interpretation of evidence, or potential obstacle that would thwart a prediction from coming true.

Thanks so much for these extremely helpful, practical advices that only a true LSAT-insider can give! Really appreciate them!!

E) does not need to be assumed because nothing in the premise/conclusion suggested the author was concerned with making a distinction between older coral and younger coral. I tend to call these common trap answers "fake comparisons". Whether older or younger coral are more susceptible to black water has no bearing on us analyzing the issue of the conclusion: whether last year's black water was or wasn't more intense than previous episodes.

I'm not sure if E "has no bearing" on the conclusion though. I feel it actually weakens the argument.
The argument says the black water last year was more intense than before, because it wiped out old corals that earlier black waters were not able to wipe out.
E actually gives an alternative explanation: the corals were of course older when attacked by last year's black water than when attacked by previous black waters, so their increased vulnerability, rather than increased intensity of the black water, could be the reason of the phenomenon.
E is actually kind of opposite of D.