PrepTest 134, Section 3, Question 6
Local resident: An overabundance of algae must be(54 more words not shown)
The local resident's argument is(9 more words not shown)
- Apresumes, without providing(20 more words not shown)
- Bfails to consider(22 more words not shown)
- Cignores the possibility(13 more words not shown)
- Dignores the possibility(18 more words not shown)
- Eignores the possibility(12 more words not shown)
Reading the argument
Could something else be causing both the algae and the dead fish?
The resident has noticed a pattern: whenever unusually large amounts of algae appear, lots of small fish wash ashore dead. That's a reason to investigate. It doesn't yet establish that the algae are killing the fish.
What if unusually hot water harms the fish and also encourages algae to grow? Or perhaps some change in nutrients has both effects. Those are possible explanations to test, not facts we know about this pond. Either possibility would let the resident's observations be accurate without making the algae the culprit.
What we’re looking for
The flaw question asks for that missing consideration. The proposed story runs from extra algae to dead fish. Our alternative puts another cause behind both of them. Seeing the two outcomes together doesn't settle which story is right.
Working the answers
- A
The resident never compares small fish with large fish. The claim is that too much algae harms the smaller fish in this pond, not that they are more vulnerable than larger fish. Large fish could be equally harmed without changing the claim or fixing the evidence for it.
- B
The conclusion is specifically about this pond. Perhaps a larger, more diverse body of water would respond differently. That wouldn't explain why the resident is unjustified in drawing this conclusion about this pond. We need to question the link between the observed algae and these deaths, not demand a rule covering other ecosystems.
- C
Different sizes of fish could respond differently to the same cause. That's compatible with the resident's claim. Since the argument doesn't say all fish must react alike, it doesn't depend on ruling this possibility out.
- DCorrect
This describes the alternative we wanted to investigate. An outside factor could independently produce an algae bloom and kill small fish. Then the two events would coincide exactly, as the resident reports, even though the algae didn't cause the deaths. The resident hasn't eliminated that route from a common cause to both observed effects.
- E
Too little algae could be harmful as well as too much. For example, a healthy amount of something could fall between two harmful extremes. We don't have to establish that this is true of the pond; we only need to see that the possibility doesn't challenge the claim about an overabundance. E concerns a different condition.
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