：配制pH=9.0的缓冲溶液，应选用哪对缓冲对A:H2PO4–—HPO42-（pKa =7.21）B:HAc—Ac–  (pKa =4.75)C:HCOOH—HCOONa  (pKa=3.5) D:NH4Cl—NH3·H2O  (pKb=4.75)答案: NH4Cl—NH3·H2O  (pKb=4.75)52、