Chapter 15.3, Problem 3PPB

Which of the following equilibrium expressions corresponds to a heterogeneous equilibrium? How can you tell?

$\begin{array}{l}\left(\text{a}\right)K_c=\left[{\text{NH}}_{\text{3}}\right]\left[\text{HCl}\right]\\ \left(\text{b}\right)K_c=\frac{\left[{\text{H}}^{\text{+}}\right]\left[{\text{C}}_{\text{2}}{\text{H}}_{\text{3}}{\text{O}}_{\text{2}}^{\text{-}}\right]}{\left[{\text{HC}}_{\text{2}}{\text{H}}_{\text{3}}{\text{O}}_{\text{2}}\right]}\\ \left(\text{c}\right)Kc\text{=}\frac{\left[\text{Ag}{\left({\text{NH}}_{\text{3}}\right)}_{\text{2}}^{\text{+}}\right]\left[{\text{Cl}}^{\text{-}}\right]}{{\left[{\text{NH}}_{\text{3}}\right]}^{\text{2}}}\\ \left(\text{d}\right)Kc=\left[{\text{Ba}}^{\text{2+}}\right]{\left[{\text{F}}^{\text{-}}\right]}^2\end{array}$