It is theoretically possible that bacteria developed on Mars early in its history and that some were carried to Earth by a meteorite.However, strains of bacteria from different planets would probably have substantial differences in protein structure that would persist over time, and no two bacterial strains on Earth are different enough to have arisen on different planets.So, even if bacteria did arrive on Earth from Mars, they must have died out.
The argument is most vulnerable to which of the following criticisms?
A.It fails to establish whether bacteria actually developed on Mars.
B.It fails to establish how likely it is that Martian bacteria were transported to Earth.
C.It fails to consider whether there were means other than meteorites by which Martian bacteria could have been carried to Earth.
D.It fails to consider whether all bacteria now on Earth could have arisen from transported Martian bacteria.
E.It fails to consider whether there could have been strains of bacteria that originated on Earth and later died out.
B，C，E 我能排除掉，答案D，但是我在 A 和D 之间纠结了很久，因为觉得D选项虽然能 weaken the argument ， 但是似乎和premise： no two bacterial strains on Earth are different enough to have arisen on different planets. 矛盾了，因为premise 已经说了没有任何两个 bacteria 能不同到起源与不同的planets，与D选项矛盾了啊， 所以我就勉强选了A。。。求牛牛点拨！！作者: taozhexiong 时间: 2012-5-25 21:11
What the author missed was the possibility that the bacteria came from Mars eliminated the bacteria from the Earth. This possibility can explain why only one type of bacteria survived on Earth while still conform to the hypothesis that some baceteria came from Mars.作者: modernre 时间: 2012-5-26 09:25
got it ! thank you ! my dear sdcar2010 ~作者: stoner28 时间: 2012-5-27 16:29
The passage already considered possibilites raised in answer choice E). If you know a bit of biochemistry, you would know that bacteria strains come and goes like day and night. Therefore if the current bacteria strains are from the Earth, then there must have been other strains from the Earth that have died.
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