Which of the following can be used to distinguish between ${\mathrm{CO}}_3^{2-}$ and ${\mathrm{HCO}}_3^{-}$?

Gaseous carbon dioxide $\left({\mathrm{CO}}_2\right)$ is a byproduct of the body's metabolism and is a waste product. Carbon dioxide is carried by the blood to the lungs, where it will be expelled. The bicarbonate $\left({\mathrm{HCO}}_3\right)$ form ${\mathrm{CO}}_2$makes up more than 90% of the gas in human blood.${\mathrm{Mg}}^{2+}+2{\mathrm{HCO}}_3^{-}⟶\mathrm{Mg}{\left({\mathrm{HCO}}_3\right)}_2 \stackrel{∆}{\to } {\mathrm{MgCO}}_3+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_2\uparrow$

$$\begin{gathered} {\mathrm{CO}}_3^{2-}+4{\mathrm{Hg}}^{2+}+3{\mathrm{H}}_2\mathrm{O}⟶{\mathrm{Hg}}_4{\mathrm{O}}_3{\mathrm{CO}}_3\downarrow \\ \text{ (Reddish brown ppt.) } \end{gathered}$$

The correct option is both (a) & (b).