Identify the buffer from the given mixtures 

50 mL 0.050 M NaF mixed with 25 mL 0.050 M NaBr

50 mL 0.050 M HF mixed with 125 mL 0.050 M KOH

50 mL 0.050 M HF mixed with 25 mL 0.050 M KBr

50 mL 0.050 M HF mixed with 25 mL 0.050 M KOH