For the redox reaction

$$\mathrm{MnO}_4^{-}+\mathrm{C}_2 \mathrm{O}_4^{2-}+\mathrm{H}^{+} \longrightarrow \mathrm{Mn}^{2+}+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O}$$,

the correct coefficients of the reactants for the balanced reaction are respectively

Assertion (A) The colour of old lead paintings can be restored by washing them with a dilute solution of $$\mathrm{H}_2 \mathrm{O}_2$$.

Reason (R) Hydrogen peroxide reduces $$\mathrm{PbS}$$ to $$\mathrm{Pb}$$.

Which among the following species acts as a self-indicator?