Conclusion In most of this forest, the expected outbreak of tree-eating tussock moths should not be countered. After all, Support the moth is beneficial where suppression of forest fires, for example, has left the forest unnaturally crowded with immature trees, and _______.
The author concludes that an outbreak of tree-eating tussock moths in a forest should not be countered in most of the forest. The author supports the conclusion by claiming that the tree-eating moths could be good for the forest in areas with too many immature trees. The question stem then asks us to fill in a missing premise to further support the conclusion.
The author assumes that the possible benefits of moths described in the argument will apply in most of the forest—in other words, that most of the forest is crowded with immature trees. The author also assumes that the moths will not have a greater detrimental effect compared to their beneficial effect.
The conclusion of the argument is most strongly supported if which one of the following completes the passage?
more than half of the forest is unnaturally crowded with immature trees
This strengthens by affirming the author’s assumption that the beneficial effects of the moth will affect most of the forest, since the problem of overcrowding that the tree-eating moths can help solve does affect more than half the forest.
mature trees are usually the first to be eaten by tussock moths
This weakens by undermining the author’s assumption that the benefits of the moths will outweigh any detrimental effects. Moths that prefer mature trees would do more harm by eating the healthy, mature trees of the forest than benefit by eating the overcrowded, immature trees.
usually a higher proportion of mature trees than of immature ones are destroyed in forest fires
This is irrelevant. First, the author has already established that a lack of forest fires still results in overcrowding of immature trees. Second, we don’t care what forest fires do—that’s just not what the argument is about, and we don’t know if this forest even had a fire.
the expected outbreak of tussock moths will almost certainly occur if no attempt is made to counter it
This is irrelevant, since we already know that not countering the outbreak will result in the outbreak occurring—we care about whether or not that’s a good thing.
there are no completely effective countermeasures against the moth
This is irrelevant to the author’s argument that the outbreak should not be countered in any way in most of the forest, regardless of whether fully effective countermeasures exist or not.
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