Which mixtures are buffer solutions?

I.     ${\text{KHS}}{{\text{O}}_{\text{4}}}{\text{(aq)}}$ and ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}{\text{(aq)}}$

II.     ${\text{C}}{{\text{H}}_{\text{3}}}{\text{COONa(aq)}}$ and ${\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH(aq)}}$

III.     ${\text{HCOOK(aq)}}$ and ${\text{HCOOH(aq)}}$

A.     I and II only

B.     I and III only

C.     II and III only

D.     I, II and III

C