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题目材料：
Scientists have long debated the exact timing of the lunar cataclysm, a period approximately 4 billion years ago when Earth and the Moon were pummeled with asteroids. A clue to this puzzle may come from spherules, millimeter-sized droplets of molten rock formed after an asteroid collides explosively with a planet. Upon impact, the asteroid vaporizes both itself and the target rock, producing a vapor plume that condenses into spherules. These form a layer preserved in rock, whose age can be estimated using radiometric dating. Scientists know of fourteen of these spherule layers scattered across Earth, but none dates to the theorized lunar cataclysm time period. Four layers. however, are from between 3.47 and 3.24 billion years ago, indicating perhaps a slow decline in collisions.
以上解析由 考满分老师提供。
