Which one of the following mixture cannot act like a buffer :

• A

  $0.1 \mathrm{M} {\mathrm{H}}_2{\mathrm{CO}}_3 + 0.01 \mathrm{M} {\mathrm{NaHCO}}_3$

• B

  0.1 M $\mathrm{KCN}$ + 0.1 M $\mathrm{HCN}$

• C

  0.1 M $\mathrm{NaCl}$ + 0.1 M $\mathrm{HCl}$       

• D

  $0.2 \mathrm{M} {\mathrm{H}}_2\mathrm{S} + 0.4 \mathrm{M} \mathrm{KHS}$