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  3. An alarming number of Mediterranean monk seals, an endangered species, have recently died. Postmortem analysis showed the presen

An alarming number of Mediterranean monk seals, an endangered species, have recently died. Postmortem analysis showed the presen

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问题     An alarming number of Mediterranean monk seals, an endangered species, have recently died. Postmortem analysis showed the presence of an as yet unidentified virus, as well as evidence of a known bacterial toxin. Seawater samples from the area where the seals died did contain unusually high concentrations of the toxic bacterium. Therefore, although both viruses and bacterial toxins can kill seals, it is more likely that these deaths were the result of the bacterial toxin.
    Which of the following, if true, provides additional evidence to support the conclusion?
选项 A、Viruses are much more difficult to identify in postmortem analysis than bacteria are.
B、Mediterranean monk seals are the only species of seal in the area where the bacterium was found.
C、The bacterium is almost always present in the water in at least small concentrations.
D、Nearly all the recent deaths were among adult seals, but young seals are far more susceptible to viruses than are adult seals.
E、Several years ago, a large number of monk seals died in the same area as a result of exposure to a different bacterial toxin.
答案D
解析 根据原文，海豹的死亡要么是由细菌导致的，要么是由病毒导致的。根据D选项，如果是病毒导致海豹死亡，那么先死的应该是小海豹，可是死的都是成年海豹，因此死亡原因必然不是病毒，那就只能是细菌导致海豹死亡了。该选项属于“排除他因”，正确。A项本文想要论证的是海豹死亡是否是由细菌导致的，与细菌和病毒谁更容易被发现无关。不选。B项原文已经说了在海豹死亡的区域发现了细菌，但至于僧海豹是否是这一区域唯一的海豹物种，与它们是否被细菌所杀没有关系，不选。C项既然细菌总是存在，那么如果真的是细菌导致海豹死亡，那么这片区域应该每天有海豹死亡。但是根据原文只有最近才发现了海豹的死亡，因此该选项反而削弱了细菌这一因素，不选。E项几年前的事情跟最近发生的事情无必然关系；不同的毒素和原文讨论的细菌毒素也没有关系，不选。
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