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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 McKayteam 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 McKayteam’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 form terrestrial contamination.However, McKay’steam 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’sstrongest argument, however, is that processes unrelated to organic life can easily produce all the evidence found by McKay’ steam, 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’steam 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.
21. The passage suggests that if a meteorite contained PAHs that were the result of terrestrial contamination, then one would except which of the following to be true?
A The meteorite would have been on Earth for more than 13,000 years.
B The meterorite would have orinated from a source other than Mars.
C The PAHs contained in the meteorite would have originated from nonbiological processes.
D The meteorite would contain fewer PHAs than most other meteorites contain.
E The PHAs contained in the meteorite would be concentrated toward the meteorite's surface.
正确答案是选E
这道题题我不太理解，还请大家指教一下~~
However, McKay’steam has demonstrated that the concentration of PAH’s increases as one looks deeper into ALH84001, contrary to what one would expect from terrestrial contamination. 这句话是本题的关键，可是我读不懂，还请NN帮我翻译一下~~ |
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发表于 2011-8-1 16:54:32
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