（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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"[In] the 1960s [it] turned out, in hindsight, we had a near-endless schedule margin there," Isaacman said. "That is certainly not the case today. I'd say this is very, very close from a timeline perspective."
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