Identify the correct statement for following reaction.

$$3 \mathrm{Mg}+\mathrm{N}_2 \longrightarrow \mathrm{Mg}_3 \mathrm{~N}_2$$

What is oxidation number of S in $\mathrm{SO}_3^{2-}$ ?

What is the change in oxidation number of S in following reaction?

$$\mathrm{H}_2 \mathrm{~S}+\mathrm{NO}_3^{-} \longrightarrow \mathrm{H}_2 \mathrm{O}+\mathrm{NO}+\mathrm{S}$$

What is the oxidation number of carbon in $\mathrm{K}_2 \mathrm{C}_2 \mathrm{O}_4 ?$