A.酚酞 B.甲基橙 C.甲基紅 D.以上三種指示劑均不行
A.NaH2PO4-Na2HPO4(pK(H2PO4-)=7.21) B.HAc-NaAc(pK(HAc)=4.76) C.HCOOH-HCOONa(pK(HCOOH)=3.75) D.NH3-NH4Cl(pK(NH3)=4.75)