15. For a triprotic acid H3A with pK₁ = 3.0, pK₂ = 6.0 and pK₃= 9.0, which of the following relations is correct?
(A) [H₃A] = [A^3- ] at pH = 7.0
(B) [HA^2- ]/[A^3-] = 10 at pH 7.0
(C) [H₂A^-] ＞ [HA^2- ] at pH 6.5
(D) [H₃A] ＞ [HA^2- ] at pH 5.0 3

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