PrepTest 148, Section 1, Question 5
During its caterpillar stage, the leopard magpie moth(50 more words not shown)
Which one of the following(7 more words not shown)
- AFeeding on the(20 more words not shown)
- BThe leopard magpie(17 more words not shown)
- CPotential predators of(17 more words not shown)
- DLeopard magpie moths(13 more words not shown)
- ENone of the(13 more words not shown)
Reading the argument
Could the moth get the same protection somewhere else? Maybe another plant makes it taste just as bad to predators. Losing the Natal grass cycad would then be less threatening than the author assumes. The proposed danger makes sense if the moth depends on this plant. Eating it provides a toxin that makes the moth unappealing to predators. But identifying one source of protection doesn't establish that it is the only source.
There's another possibility too: perhaps predators have learned to avoid these moths and wouldn't notice that the toxin was gone. The moths could keep benefiting from that learned avoidance even after losing the substance that originally made them unpleasant to eat.
What we’re looking for
For a required assumption, we want something that defends the argument against a possibility like one of those. We aren't being asked to introduce an objection; we're looking for a claim the author needs in order to move from the endangered plant to the endangered moth.
Working the answers
- ACorrect
This blocks the alternative-source objection. If eating the cycad is the only way the moth can make itself highly unpalatable, it can't simply switch to another plant or another way of acquiring the same protection. The argument depends on the threat to this plant also threatening that defense. A supplies that dependence; it doesn't have to eliminate every other imaginable way the moth might survive.
- B
The moth needn't be unable to escape every potential predator. Suppose it could escape one of ten kinds but remained vulnerable to the other nine. The loss of its unpleasant taste could still put it in danger. B rules out any escape ability at all, which is much more than the argument requires.
- C
This could fit the learned-avoidance possibility: predators might keep avoiding moths because they can't see that the toxin is missing. But that is a possible challenge to the author's prediction, not a claim needed to defend it. The argument doesn't require predators to be unable to judge the toxin's presence by appearance alone.
- D
Rarity doesn't automatically make the plant impossible for the moth to find. Imagine moths that could locate rare plants or only needed an occasional feeding to gain protection. They could still face a threat if the plant disappeared altogether. The argument doesn't require the stronger claim that the moth can't locate the plant unless it is abundant.
- E
Some predators could tolerate the toxin while others still avoid it. The plant could remain an important defense against those other predators, and losing that defense could threaten the moth. The argument doesn't need every potential predator to lack tolerance; it needs the endangered plant to supply protection the moth cannot simply replace, as A states.
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