Among all the isomers of molecular formula $\mathrm{C}_4\mathrm{H}_9\mathrm{Br}$, identify (a) the one isomer which is optically active. (b) the one isomer which is highly reactive towards $\mathrm{S}_{\mathrm{N}}^2$. (c) the two isomers which give same product on dehydrohalogenation with alcoholic $\mathrm{KOH}$