质子数和中子数相同的原子A,其阳离子An+核外共有x个电子,则A的质

质子数和中子数相同的原子A,其阳离子An+核外共有x个电子,则A的质量数为(    )

A.2(x-n)         B.2(x+n)             C.2x             D.n+2x

答案

B

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