Which substance cannot be reduced by ${\mathrm{H}}_2{\mathrm{O}}_2$

• A

  ${\mathrm{KMnO}}_4/{\mathrm{H}}_2{\mathrm{SO}}_4$

• B

  ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7/{\mathrm{H}}_2{\mathrm{SO}}_4$

• C

  ${\mathrm{Ag}}_2\mathrm{O}$

• D

  ${\mathrm{Fe}}^{3+}$