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  3. Typically during thunderstorms most lightning strikes carry a negative electric charge; only a few carry a positive charge. Thun

Typically during thunderstorms most lightning strikes carry a negative electric charge; only a few carry a positive charge. Thun

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问题 Typically during thunderstorms most lightning strikes carry a negative electric charge; only a few carry a positive charge. Thunderstorms with unusually high proportions of positive-charge strikes tend to occur in smoky areas near forest fires. The fact that smoke carries positively charged smoke particles into the air above a fire suggests the hypothesis that the extra positive strikes occur because of the presence of such particles in the storm clouds.
Which of the following, if discovered to be true, most seriously undermines the hypothesis?
选项 A、Other kinds of rare lightning also occur with unusually high frequency in the vicinity of forest fires.
B、The positive-charge strikes that occur near forest fires tend to be no more powerful than positive strikes normally are.
C、A positive-charge strike is as likely to start a forest fire as a negative-charge strike is.
D、Thunderstorms that occur in drifting clouds of smoke have extra positive-charge strikes weeks after the charge of the smoke particles has dissipated.
E、The total number of lightning strikes during a thunderstorm is usually within the normal range in the vicinity of a forest fire.
答案D
解析 Argument Evaluation
Situation Thunderstorms with unusually high proportions of positive-charge lightning strikes tend to occur in smoky areas near forest fires. Smoke carries positively charged particles into the air above fires, suggesting that smoke particles in storm clouds are responsible for the higher proportion of positive strikes.
Reasoning What would cast doubt on the hypothesis that the extra positive-charge lightning strikes in thunderstorms near forest fires result from positively charged smoke particles carried into the storm clouds? The hypothesis would be weakened by evidence that the positively charged smoke particles do not enter the storm clouds in the first place, or that they do not retain their charge in the clouds long enough to produce an effect, or that their positive charge cannot affect the charges of the storm’s lightning strikes in any case, or that some other factor tends to make the lightning strikes above these storms positively charged.
A It could be that positively charged smoke particles cause these other kinds of rare lightning, too, so this does not seriously undermine the hypothesis.
B The hypothesis is not about the power of the positive-charge lightning strikes, only about why a high proportion of them occur in thunderstorms near forest fires.
C The hypothesis is about why positive-charge strikes tend to occur in smoky areas near forest fires that have already started before the strikes occur. Furthermore, an equal likelihood of positive-charge and negative-charge strikes starting fires cannot explain a correlation between fires and positive-charge strikes specifically.
D Correct. This means that even when drifting clouds of smoke persist for weeks after a fire, when the charge of their particles has already dissipated, the smoke somehow still makes the strikes positively charged in any thunderstorms arising within it. If so, some factor other than positively charged smoke particles must affect the strikes’ charge.
E This information does not undermine the hypothesis. The hypothesis does not concern the possibility that there might be more lightning strikes in the vicinity of forest fires; rather it concerns the proportion of all such lightning strikes that are positively charged.
The correct answer is D.
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