18. What ionic forms are present at pH 7.0? The pKas of phosphoric acid are 2.1, 7.2, 12.4.
(A) HPO₄^-2
(B) H₂PO₄^-， 
(C) HPO₄^-2 and PO₄^-3
(D) H₂PO₄^- and HPO₄^-2
(E) All are correct

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