单项选择题
A.HCl-NaClB.CH3COOH-CH3COONaC.NH3·H2O-NH4ClD.H2CO3-NaHCO3E.NaH2PO4-Na2HPO4
A.NB.N2C.OD.Mg2+E.NH4+
A.KB.ClC.NaD.FE.Ca