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题目材料：
The traditional picture of planet formation starts with small grains that collide and build into boulders, then larger and larger objects. If this is correct, the Kuiper Belt, an outer region of the solar system that exists beyond the planets, should hold leftover planetary building blocks. But that idea is undermined by Jupiter`s moon Europa. One would expect that over millions of years Europa could have been repeatedly impacted by small comets formed from leftover planetary material in the Kuiper Belt. In fact, Europa`s surface reveals very few small comet impact craters. Perhaps the comets disappear when they enter the inner solar system and are heated by the Sun. Or perhaps the Kuiper Belt contains no leftover planetary material from which comets could have formed.
以上解析由 考满分老师提供。
