（六）放射性同位素，是指化学元素中具有相同原子序数，但质量不同且能发生放射性衰变的核素。
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"To some people, this is tantamount to, 'we own this bit of the moon, we're going to operate here and and you can't come in'," he explains.
第二重考验，是用户体验的打磨。