The dibromo compound $[P]$ (molecular formula: $\mathrm{C}_9 \mathrm{H}_{10} \mathrm{Br}_2$ ) when heated with excess sodamide followed by treatment with dilute HCl gives $[Q]$. On warming $[Q]$ with mercuric sulphate and dilute sulphuric acid yield $[R]$ which gives positive lodoform test but negative Tollen's test. The compound $[P]$ is :