For the reversible system $\mathrm{PCl}_3+\mathrm{Cl}_2 \rightleftharpoons \mathrm{PCl}_5$, the relationship between degree of dissociation $(\alpha)$ of $\mathrm{PCl}_5$ and the equilibrium constant $\mathrm{K}_{\mathrm{p}}$ of the above equilibrium is
(a) $\frac{\mathrm{K}_{\mathrm{p}}}{\sqrt{\mathrm{P}+\mathrm{K}_{\mathrm{p}}}}$
(b) $\sqrt{\frac{K_p}{1+P \cdot K_p}}$
(c) $\frac{1}{\sqrt{\mathrm{P}+\mathrm{K}_{\mathrm{p}}}}$
(d) $\sqrt{\frac{1}{1+\mathrm{P} \cdot \mathrm{K}_{\mathrm{P}}}}$