单项选择题
Scientists have long sought the reasons for the relatively young age of the majority of Martian meteorites discovered on Earth 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 to Earth 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 on Earth. 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 reach Earth are predictably biased toward younger ages.Scientists have long sought the reasons for the relatively young age of the majority of Martian meteorites discovered on Earth 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 to Earth 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 on Earth. 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 reach Earth are predictably biased toward younger ages.
The passage is primarily concerned with（）. A．presenting an argument to support a particular hypothesis
B．suggesting an answer to a theoretical question
C．questioning the assumptions of a research project
D．criticizing experimental results
E．explaining the origin of certain scientific data