A given nitrogen-containing aromatic compound $\mathrm{A}$ reacts with $\mathrm{S}\mathrm{n}/\mathrm{H}\mathrm{C}\mathrm{l}$, followed by $\mathrm{H}\mathrm{N}{\mathrm{O}}_2$ to give an unstable compound $\mathrm{B}$. $\mathrm{B}$ on treatment with phenol, forms a beautiful coloured compound $\mathrm{C}$ with the molecular formula ${\mathrm{C}}_{12}{\mathrm{H}}_{10}{\mathrm{N}}_2\mathrm{O}$. The structure of compound $\mathrm{A}$ is