PrepTest 137, Section 4, Question 23
Biologists found that off the northeast coast of(66 more words not shown)
Which one of the following,(9 more words not shown)
- ASeveral major pharmaceutical(18 more words not shown)
- BA new strain(23 more words not shown)
- CA powerful toxin(18 more words not shown)
- DFish species X,(25 more words not shown)
- EGlobal warming has(15 more words not shown)
Reading the argument
What could hurt these particular organisms?
A drop in P-plankton could have many explanations: weather, pollution, or a predator. The fish deaths add another fact to explain. Because X, Y, and Z sometimes eat P-plankton, it is tempting to connect the deaths to a loss of food. But 'sometimes eat' doesn't establish dependence on that food, and a 10 percent decline in plankton doesn't automatically explain extraordinarily high fish death rates.
There might be a connection, but we need a mechanism. Perhaps a chemical affects all four kinds of organisms. Perhaps a predator attacks them. A shared cause could connect the changes without requiring the plankton decline itself to cause the fish deaths.
What we’re looking for
The question asks us to explain the findings. Keep the entire pattern in view: less P-plankton, high death rates in these three fish species, and no apparent effect on other species. An explanation that handles only the fish or only the plankton leaves part of the puzzle open.
Working the answers
- A
Dumped waste is a possible place to investigate, but we aren't told what it does to these organisms. The dumping has also been happening for many years, while the population changes are recent. We'd need more information to connect that longstanding activity to this particular new pattern.
- BCorrect
The new bacterium attacks both groups: it destroys P-plankton cell walls and attacks the respiratory systems of X, Y, and Z. That supplies a common cause of the plankton decline and the fish deaths.
The damage doesn't need to happen in the same way. A single cause can harm different organisms through different mechanisms. The connection is the bacterium, not a claim that damaged plankton cell walls somehow cause fish respiratory problems.
B doesn't explicitly establish that the bacterium can affect no other species. But it directly identifies a cause attacking the specific organisms with the reported problems, without requiring us to invent those effects. That makes it the strongest explanation offered.
- C
This gives us a reason for the plankton decline, but it doesn't explain the unusually high fish death rates. We would still have to connect the loss of plankton to the fish deaths, or supply another effect of the toxin. B explicitly accounts for both.
- D
This is tempting because it connects less plankton with more fish deaths. But why did the P-plankton population drop? D doesn't explain that part of the findings. It also starts with widespread fish starvation already underway. It describes how the plankton loss worsens that problem without supplying a common explanation for the whole pattern.
- E
Changed climatic conditions might affect ocean life, but the answer never tells us how they affect these organisms in particular. We would have to supply a link to both the plankton decline and the selective fish deaths. A general environmental change isn't as explanatory as B's specific attacks on the affected groups.
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