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两枚同样大小的硬币,其中一个固定,另一个沿其周围滚动,滚动时,两枚硬币始终保持有一点接触,则滚动硬币再回到原来位置时,它自转的周数为( ).
Two coins of the same size, one of which is fixed and the other is rolling along its circumference. When rolling, the two coins always have a slight contact. When the coin is rolled back to its original position, the number of revolutions is ().