甲、乙是周期表中同一主族的两种元素，若甲的原子序数为x，则乙的原子序数不可能是

1. A.
   x+2
2. B.
   x+4
3. C.
   x+8
4. D.
   x+18

A.x+2                 B.x+4                 C.x+8                D.x+18

A.x+2                    B.x+4                    C.x+8                      D.x+18

A.x+2                  B.x+4                     C.x+8                    D.x+18