PrepTest 107, Section 4, Question 20
Amphibian populations are declining in numbers worldwide.(82 more words not shown)
Each of the following, if(6 more words not shown)
- AOf the various(16 more words not shown)
- BAmphibian populations are(21 more words not shown)
- CAtmospheric ozone has(16 more words not shown)
- DThe natural habitat(10 more words not shown)
- EAmphibian populations have(7 more words not shown)
Reading the argument
The frogs are declining, and the ozone layer is thinning. Are those events connected, or could something else be affecting both?
Perhaps pollution contributes to both changes. Or perhaps habitat loss or a warming environment explains much of the amphibian decline. Those are possibilities to investigate; matching trends alone do not establish the proposed cause.
The author does give us more than matching trends. Ozone blocks UV-B radiation, which can damage genes, and amphibians lack much of the physical protection other animals have. Their eggs are vulnerable too. Think of the ozone layer as a protective screen: less protection could expose these animals to more damage. That makes the proposed mechanism plausible.
What we’re looking for
But is ozone depletion the primary cause? Showing how it could contribute does not establish that it matters more than the alternatives. For this strengthen-EXCEPT question, four answers will give the explanation some support. Evidence against another cause or a closer match between ozone depletion and amphibian decline could help. We want the one that adds no such support.
Working the answers
- ACorrect
This can sound helpful because it singles out UV-B as the only gene-damaging radiation blocked by ozone. But the conclusion identifies ozone depletion as the primary cause of the population decline. It does not claim UV-B is the primary cause rather than other kinds of radiation.
If another gene-damaging form of radiation were also blocked by ozone, that would give another possible way ozone depletion could harm amphibians. Ruling that out does not make ozone depletion more likely to be the primary cause. We already know UV-B can damage genes. A adds a comparison among radiation types without strengthening the needed comparison between ozone depletion and other possible causes of the decline.
- B
This supports the proposed mechanism. The less-protected amphibians are declining faster than species with more natural protection from UV-B. That is a pattern we would expect if exposure to UV-B contributes to the decline. It does not prove the cause, but it makes the vulnerability described in the passage more relevant.
- C
This puts the proposed cause where the decline occurs. Significant ozone depletion above all the affected areas strengthens the connection between ozone loss and amphibian losses. It would be harder to explain the decline through ozone depletion if affected places did not experience that depletion.
- D
This weakens one alternative we considered: shrinking habitat. If amphibians' natural habitat has not become smaller, habitat loss is less promising as an explanation of their falling numbers. Removing a competing explanation gives the proposed cause some support without proving it is the main cause.
- E
This adds a matching time pattern. The ozone layer has depleted continuously for 50 years, and amphibian populations have declined continuously over those same years. That is modest support, not proof of causation. But an additional match in timing still helps the argument, so E is not the exception.
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