PrepTest 152, Section 1, Question 13
When so many oysters died off the coast(72 more words not shown)
Which one of the following,(15 more words not shown)
- AThe increase in(12 more words not shown)
- BNative oysters rely(16 more words not shown)
- CTBT also killed(18 more words not shown)
- DOther chemicals that(19 more words not shown)
- ETBT is more(10 more words not shown)
Reading the argument
Removing the poison doesn't instantly undo the damage.
My first thought is that the oyster population may already have been devastated. Stopping further poisoning doesn't bring dead oysters back; the survivors would still have to rebuild the population.
I'd also wonder whether the chemical remains, but we have to respect the statement that it has been nearly eliminated. We can't explain the puzzle simply by pretending the old concentration is still there.
What we’re looking for
So why haven't these native populations grown after the ban? The answer needs to identify something still preventing recovery. Let's see what the choices add.
Working the answers
- A
A slower increase in temperature doesn't explain why the populations remain unable to recover. The passage identified TBT as the cause of the die-off. Temperature might matter through some further mechanism, but A doesn't supply one that keeps the native oysters from growing in number.
- B
The barnacles aren't said to be taking food away from the oysters. The problem was the chemical used to remove barnacles from boats. Telling us that barnacles and native oysters eat different things doesn't explain what stops the oysters from recovering once that chemical is nearly gone.
- CCorrect
There's a different obstacle: competing oysters. TBT had also been killing imported varieties. Once the chemical is removed, those varieties can flourish in the warmer water at the native oysters' expense. Removing TBT therefore frees the natives from one threat while also freeing their competitors. The competition can keep the native populations from growing.
This isn't my initial idea about damage already done or lingering chemical. It explains the same puzzle through a new problem created by removing something that had been suppressing both groups.
- D
A harmful replacement chemical could explain the failed recovery. If the boats simply switched to another substance that also killed oysters, the ban might not help much. But D tells us these other chemicals seem to have little effect. That fails to supply the damaging replacement we would need.
- E
This compares TBT's harmfulness at different temperatures. It doesn't show that enough TBT remains after the ban to prevent recovery. The chemical has been nearly eliminated, so we need more than a statement about which temperature would make it more deadly if it were present.
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