15. 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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