For the reaction $A+B =C+D$ , the equilibrium can be expressed as :

Question

$\begin{array}{1 1} (a)\; K=\large\frac{[A][B]} {[C][D]}\\ (b)\; K=\large\frac{[C][B]} {[C][D]} \\ (c)\;K=\large\frac{[C][D]} {[A][B]} \\ (d)\;K=\large\frac{[A][C]} {[B][D]} \end{array} $