For the equilibrium $2 H _{2} O ( g ) \rightleftharpoons 2 H _{2}( g )+ O _{2}( g )$, equilibrium constant is $K _{1}$. For the equilibrium $2 CO _{2}( g ) \rightleftharpoons 2 CO ( g )+ O _{2}( g )$, equilibrium constant is $K _{2}$. Then, the equilibrium constant for $CO _{2}( g )+ H _{2}( g ) \rightleftharpoons CO ( g )+ H _{2} O ( g )$ is :
(A) $K_{1} K_{2}$
(B) $\frac{K_{1}}{K_{2}}$
(C) $\sqrt{\frac{ K _{1}}{ K _{2}}}$
(D) $\sqrt{\frac{ K _{2}}{ K _{1}}}$