For the reversible reaction, A+B⇌C+D  taking place in one litre vessel at 450 K, initial moles of A and B are equal. If the number of moles of A at equilibrium are double the number of moles of C at equilibrium then the equilibrium constant of the reaction is equal to

A +   B  ⇌  C +  D; Initial moles           1        1          -     -; Moles at Eqbm 1-x 1-x      x       x; Given                       1-x=2x;                             x=13; ∆n=0 for the process, volume of the vessel has no effect in the calculation of  Kc; Kc=x21-x2=0.25