9. Which of these is an isoelectronic series?
(A) Na⁺, K⁺, Rb⁺, Cs⁺
(B) K⁺, Ca²⁺, Ar, S^2–
(C) Na⁺, Mg²⁺, S^2–, Cl^–
(D) Li, Be, B, C
(E) none of these (A-D)

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