In the dissociation of \[PC{{l}_{5}}\] as \[PC{{l}_{5}}(g)\rightleftharpoons PC{{l}_{3}}(g)+C{{l}_{2}}(g)\] if the degree of dissociation is a at equilibrium pressure P, then the equilibrium constant for the reaction is

• A. \[{{K}_{P}}=\frac{{{\alpha }^{2}}}{1+{{\alpha }^{2}}P}\]  
• B. \[{{K}_{P}}=\frac{{{\alpha }^{2}}{{P}^{2}}}{1-{{\alpha }^{2}}}\]
• C. \[{{K}_{P}}=\frac{{{P}^{2}}}{1-{{\alpha }^{2}}}\]
• D. \[{{K}_{P}}=\frac{{{\alpha }^{2}}P}{1-{{\alpha }^{2}}}\]