A hydrocarbon $\mathrm{X}$ with molecular formula ${\mathrm{C}}_4{\mathrm{H}}_6$ decolorizes bromine water and forms a white  precipitate in ethanolic ${\mathrm{AgNO}}_3$ solution Treatment of $\mathrm{X}$ with ${\mathrm{HgCl}}_2$ in aqueous ${\mathrm{H}}_2{\mathrm{SO}}_4$ produces a compound, which gives a yellow precipitate when treated with ${\mathrm{I}}_2$ and $\mathrm{NaOH}$. The structure of $\mathrm{X}$ is :