Which of the following options is correct?

(P) ${\mathrm{Hg}}_2{\mathrm{Cl}}_2$ can be used for the distinction of ${\mathrm{HCO}}_3^{-}$  and ${\mathrm{CO}}_3^{2-}$

(Q) ${\mathrm{Hg}}_2{\mathrm{Cl}}_2$ is insoluble in water.

(R) Aq. suspension of ${\mathrm{Ag}}_2{\mathrm{CO}}_3$ and ${\mathrm{Ag}}_2{\mathrm{SO}}_3$ both produces ${\mathrm{Ag}}_2\mathrm{O}$ on heating.

(S) ${\mathrm{SO}}_2$ acts as reducing agent when passed through ${\mathrm{FeCl}}_3$ solution.