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Recently some scientists have concluded that meteorites found on Earth and long believed to have a Martian origin might actually have been blasted free of Marss gravity by the impact on Mars of other meteorites. This conclusion has led to another question: whether meteorite impacts on Earth have similarly driven rocks from this planet to Mars. According to astronomer S. A. Phinney, kicking a rock hard enough to free it from Earths gravity would require a meteorite capable of making a crater more than 60 miles across. Moreover, even if Earth rocks were freed by meteorite impact, Marss orbit is much larger than Earths, so Phinney estimates that the probability of these rocks hitting Mars is about one-tenth as great as that of Marss rocks hitting Earth. To demonstrate this estimate, Phinney used a computer to calculate where 1,000 hypothetical particles would go if ejected from Earth in random directions. He found that 17 of the 1,000 particles would hit Mars. 17. The passage is primarily concerned with presenting an argument to support a particular hypothesis suggesting an answer to a theoretical question questioning the assumptions of a research project criticizing experimental results explaining the origin of certain scientific data 18. According to the passage, which of the following events may have initiated the process that led to the presence on Earth of meteorites from Mars? | 
