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题目材料：
One difficulty in convincing early scientists that craters formed as a result of impacts from space is that most craters are circular. Impacts could come in at any angle, and experiments firing projectiles in the laboratory show that low-angle impacts lead to elliptical craters, not circular ones. Furthermore,while there was rarely evidence of any impacting object,there was often silicate melt around,suggesting that craters were caused by volcanic processes. The breakthrough in understanding crater origin was the recognition that the shock caused by the impacting object-not the object itself-creates a circular crater some twenty times larger than the diameter of the impactor.The impact also generates enough heat to largely vaporize the impactor and melt the native rock.
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