In the reversible reaction shown below, $k_{f}$ is the forward reaction rate constant and $k_{r}$ is the reverse reaction rate constant.
The CORRECT expression representing the reaction equilibrium constant is $\_\_\_\_$.
$$A+B \underset{k_{r}}{\stackrel{k_{f}}{\rightleftharpoons}} C+D$$
- $k_{f}+k_{r}$
- $k_{f} \times k_{r}$
- $\dfrac{k_{f}}{k_{r}}$
- $\dfrac{k_{r}}{k_{f}}$
