已知元素X、Y的核电荷数分别是a和b，它们的离子Xm+和Yn-的核外电子排布相同，则下列关系式正确的是（ ）
| A．a =" b+m" +n | B．a =" b-m" +n | C．a = b+m-n | D．a = b-m-n |
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