PrepTest 142, Section 2, Question 10
In a vast ocean region, phosphorus levels have(57 more words not shown)
Which one of the following(8 more words not shown)
- AThe agricultural runoff(15 more words not shown)
- BBefore phosphorus levels(14 more words not shown)
- CIf agricultural runoff(18 more words not shown)
- DThe quantity of(14 more words not shown)
- EThe amount of(21 more words not shown)
Reading the argument
More phosphorus sounds important, but what does it do? Follow the explanation one step at a time.
Agricultural runoff brings phosphorus into the ocean region. The phosphorus stimulates plankton growth. When plankton decay and fall, bacteria on the ocean floor eat them and consume oxygen. That leaves less oxygen available, making the region difficult for fish to survive in.
What we’re looking for
We have a chain of facts to use, not an unsupported conclusion to attack. An inference could connect the runoff with something farther along that chain. It needn't mention every stage or reach all the way to the fish. We just need to keep the connection within what the passage establishes.
Working the answers
- ACorrect
Runoff raises phosphorus levels, and phosphorus stimulates plankton growth. Those two links establish that the runoff contributes to plankton growth. A doesn't have to mention bacteria, oxygen, or fish; connecting the first part of the explanation is enough.
- B
Knowing that few fish can survive now doesn't tell us that most could survive before phosphorus levels doubled. The region could already have been unsuitable for many fish. The passage gives us a present problem and its cause, but doesn't quantify how hospitable the region was previously.
- C
Stopping the runoff doesn't guarantee that all bacterial consumption of decaying plankton stops. Plankton might still grow for other reasons, and bacteria might continue consuming what is already there. The passage identifies something contributing to the process without saying that the process depends entirely on continued agricultural runoff.
- D
The phosphorus level doubled; the amount of runoff isn't given. Imagine the same volume of water carrying fertilizer with more phosphorus in it. Phosphorus could increase without the runoff volume doubling. We aren't entitled to transfer the numerical change in one thing to another merely because the two are connected.
- E
This proposes a general mathematical relationship between phosphorus and oxygen in bodies of water. The passage describes a particular region and a process involving plankton and bacteria. It neither extends that process to water in general nor specifies a proportional rate of change. Another body of water might lack the conditions needed for the same process. E goes beyond both the scope and the precision of the evidence.
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