GRE课程
Questions 1 and 2 are based on this passage
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.
Consider each of the choices separately and select all that apply.
Which of the following might plausibly account for the findings in the highlighted sentence?
Spherule layers older than 3.47 billion years exist, but they have not been discovered yet.
Spherule layers older than 3.47 billion years once existed, but they have since been destroyed.
Fewer asteroids collided with Earth than with the Moon during the lunar cataclysm.
Select one or more answer choices.
