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The argument challenges the scientists’ claim that industrial pollution is responsible for the decline in certain species of amphibians. To do so, the author proposes an alternative cause: natural variations in the weather could just as easily be the cause, because such variations cause most amphibian species’ populations to vary greatly from year to year.
- Scientists: Industrial pollution (cause) Decline in population of species A (effect)
Author: Variations in weather (cause) Population fluctuation of most species (effect)
At first glance, the argument makes sense: alternative causes can, and do, undermine claims of causation. Read a bit more closely, however, and you’ll realize that the author’s counterpremise applies to most, but not necessarily all, amphibian species. The scientists, by contrast, are only worried about certain specific species of amphibians. Are these among the species whose populations vary greatly as a result of natural variations in the weather? Hard to say. This is the real issue with this argument—if you spotted it, answering this question would be relatively straightforward.
Because this is an assumption question, the answer you select must contain a statement upon which the argument depends, i.e. a statement that is necessary for the conclusion to be true. Remember to apply the Assumption Negation Technique when examining your contenders: the correct answer choice, when logically negated, must definitively disprove the conclusion.
Answer choice (A): This is the correct answer choice. Due to the convoluted nature of this statement, your best—and perhaps only—approach is to negate the statement and ask yourself if the negation would weaken the author’s conclusion:
The amphibian species that scientists are worried about are among those species whose populations do not vary greatly as a result of natural variations in the weather.If true, then the author’s counterpoint does not apply to the species in question, and the natural variations in the weather could not be responsible for their decline. Since the logical opposite of answer choice (A) weakens the author’s conclusion, answer choice (A) contains an assumption necessary for the conclusion to be true.
Answer choice (B): The author makes no assumptions about the relative impact of the two causes being discussed. If declines due to weather were just as large as those due to industrial pollution, that would actually strengthen, not weaken, the conclusion of the argument.
Answer choice (C): This is an attractive, but incorrect, answer choice. The author concludes that we cannot be sure which is the correct cause, not that there is only one possible cause. The causes need not be exclusive. Indeed, even if both factors—pollution and the weather—contributed to the amphibian population’s decline, that would still be consistent with the views expressed in the conclusion.
Answer choice (D): This answer choice can easily be eliminated on the basis of its language alone. The argument makes no assumptions about reversing the decline in the future. The issue at stake is what caused the populations of amphibians to decline in the past.
Answer choice (E): Whether one of the two possible causes can actually exacerbate the other is impossible to prove, and is hardly necessary for the argument’s conclusion to be logically valid.