PrepTest 137, Section 3, Question 11
A mass of "black water" containing noxious organic(71 more words not shown)
Which one of the following(7 more words not shown)
- AMasses of black(19 more words not shown)
- BEvery species of(18 more words not shown)
- CThe mass of(22 more words not shown)
- DThe mounds of(20 more words not shown)
- EOlder specimens of(19 more words not shown)
Reading the argument
Those coral mounds survived for more than two centuries. I can see the appeal of the argument: if equally intense black water had struck before, wouldn't it have killed them then?
But was intensity the whole reason they died this time?
Imagine black water of a particular intensity passing through quickly on one occasion but lingering on another. Or imagine it arriving during unusually cold weather that adds to the stress on the coral. These are possibilities to investigate, not facts about Laurel Bay. They show why the same intensity might produce different damage under different conditions.
The author uses survival in earlier years as evidence about the water's earlier intensity. That comparison needs the coral's survival to be a reliable guide. If something else made the latest event more destructive, the old mounds could have survived equally intense water in the past.
What we’re looking for
For the necessary-assumption question, look for a claim that protects that comparison from a relevant alternative. We don't need a complete history of the bay. We need to see what the author relies on when using these mounds as a record of past intensity.
Working the answers
- A
The argument leaves open whether black water struck the bay before. It says that even if it did, it wasn't as intense. The phenomenon could occur less often than once every two centuries without undermining that conclusion. A adds a frequency the reasoning doesn't require.
- B
Five species were wiped out, including the old mounds. We don't need every coral species in the bay to have been seriously harmed. Other species could be more resistant while the old mounds still supply the evidence on which the author relies.
- C
What happened to plants or animals that use coral doesn't settle whether the mounds' past survival establishes lower water intensity. Those other organisms could have been harmed too. The argument doesn't require that they escaped destruction.
- DCorrect
What if the old mounds were especially fragile just before last year's event? Water of an intensity they had survived earlier could now kill them. Their earlier survival would no longer establish that past black water was less intense.
The argument needs to rule out that unusual vulnerability when drawing its comparison. D removes this particular alternative; it doesn't by itself prove every part of the conclusion.
- E
We don't need an old-versus-young difference in vulnerability. Both could have been equally vulnerable, and the author could still reason from the old mounds' long survival. The relevant comparison is these mounds across time, not older specimens against younger ones.
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