单项选择题

欲配制pH=4.6和pH=7.0的缓冲溶液,应先用()(已知:KaΘ(HCOOH)=1.8×10–4,KaΘ(HAc)=1.8×10–5,KbΘ(NH3)=1.8×10–5,H2CO3的Ka2Θ=5×10–11,H3PO4的Ka2Θ=1.0×10–7

A.HCOOH—HCOONa和NaHCO3—Na2CO3
B.HAc—NaAc和NaH2PO4—NaHPO4
C.HCOOH—HCOONa和NH3—NH4Cl 
D.HAc—NaAc和NH3—NH4Cl