In 1994, a team of scientists led by David Mckay began studying the meteorite ALH84001, which had been discovered in Antarctica in 1984. Two years later, the McKay team announced that ALH84001, which scientists generally agree originated on Mars, contained compelling evidence that life once existed on Mars. This evidence includes the discovery of organic molecules in ALH84001, the first ever found in Martian rock. Organic molecules—complex, carbonbased compounds—form the basis for terrestrial life. The organic molecules found in ALH84001 are polycyclic aromatic hydrocarbons, or PAH’s. When microbes die, their organic material often decays into PAH’s.
Skepticism about the McKay team’s claim remains, however. For example, ALH84001 has been on earth for 13,000 years, suggesting to some scientists that its PAH’s might have resulted from terrestrial contamination. However, McKay’s team has demonstrated that the concentration of PAH’s increases as one looks deeper into ALH84001, contrary to what one would expect from terrestrial contamination. The skeptic’s strongest argument, however, is that processes unrelated to organic life can easily produce all the evidence found by McKay’ s team, including PAH’s. For example, star formation produces PAH’s. Moreover, PAH’s frequently appear in other meteorites, and no one attributes their presence to life processes. Yet McKay’s team notes that the particular combination of PAH’s in ALH84001 is more similar to the combinations produced by decaying organisms than to those originating form nonbiological processes.
Which of the following best describes the function of the last sentence of the first paragraph?
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第一段最后一句话的作用 When microbes die, their organic material often decays into PAH’s.是说当微生物死了之后会腐化产生PAH，也就是说了PAH的来源
选项A 正确
选项B a feature of PAHs 这里讨论的不是PAH的特征，还是来源，而且other types of organic molecules改句未提及
选项C a characteristic common to most meteorites originates.大多数陨石来源特征，错误，讨论的不是陨石
选项D terrestrial contamination未提及
选项E undermines错误，或者说反了，微生物腐化可以产生PAH，说明这个陨石上的PAH大概也是微生物腐化产生的，微生物也是生命，支持火星上曾经存在生命。
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