GRE考试

【单选题】Scientists have long sought the reasons for the relatively young age of the majority of Martian meteorites discovered onEarth in comparison with the age of Mars, an enigma exarcebated by the fact that the Martian rocks were ejected Line by only six or seven separate impact events. Previous tests had predicted that driving meteorites toEarth would require a collision with an asteroid immense enough to make a crater 12-kilometers across, but because such huge impacts are extremely infrequent, it was unlikely that enough of them could have occurred to explain our planet’’s Martian meteorite collection. Now astronomer James Head’’s higher-resolution models demonstrate that collisions making craters only three kilometers across can jettison 10 million fragments, each about 10 centimeters across, into space, a distribution sufficient to cause some of them to be found onEarth. Sections of the planet covered by debris (thus likely to be made up of older terrain) would require larger and hence rarer impacts, and thus meteorites which reachEarth are predictably biased toward younger ages.Scientists have long sought the reasons for the relatively young age of the majority of Martian meteorites discovered onEarth in comparison with the age of Mars, an enigma exarcebated by the fact that the Martian rocks were ejected Line by only six or seven separate impact events. Previous tests had predicted that driving meteorites toEarth would require a collision with an asteroid immense enough to make a crater 12-kilometers across, but because such huge impacts are extremely infrequent, it was unlikely that enough of them could have occurred to explain our planet’’s Martian meteorite collection. Now astronomer James Head’’s higher-resolution models demonstrate that collisions making craters only three kilometers across can jettison 10 million fragments, each about 10 centimeters across, into space, a distribution sufficient to cause some of them to be found onEarth. Sections of the planet covered by debris (thus likely to be made up of older terrain) would require larger and hence rarer impacts, and thus meteorites which reachEarth are predictably biased toward younger ages. Which of the following, if true, would cast most doubt on the conclusions drawn from Head’’s higher-resolution models
A、The results of geographical survey prove that no crater larger than twelve kilometers exists on the surface of Mars.
B．Fragments from smaller impact events on Mars are highly unlikely to survive entry through theEarth’’s atmosphere.
C．The velocity of rocks escaping from Mar’’s gravity is greater when they have been loosened by a large impact.
D．Huge impacts on Mars are proven to be less rare than was previously thought.
E．
A、double-succession of large impacts is found to have occurred on Mars in a relatively short period of time.

网考网参考答案：B
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