4. 將下列各溶液混合，何者可以形成緩衝溶液？
(A)0.2M CH3COOH 50mL＋0.2M CH3COONa 50mL
(B)0.2M CH3COOH 100mL＋0.1M NaOH 50mL
(C)0.3M CH3COONa 100mL＋0.1M HCl 50mL
(D)0.5M NH3 20mL 加水稀釋至 100mL＋0.5M HCl 10mL
(E)0.5M NH4Cl 30mL 加水稀釋至 100mL＋0.5M NaOH 10mL。