For the dissociation $\mathrm{PCl}{}_5(\mathrm{g})\rightleftharpoons \mathrm{PCl}{}_3(\mathrm{g})+\mathrm{Cl}{}_2(\mathrm{g})$, let $D$ be the vapour density calculated for undissociated $\mathrm{PCl}{}_5$ and $d$ the vapour density of the equilibrium mixture at the same temperature and pressure. The degree of dissociation $\alpha$ is

1. A$\frac{D}{D-d}$
2. B$\frac{d-D}{2d}$
3. C$\frac{D-d}{d}$
4. D$\frac{D-d}{D}$