Which one of the following is an example of heterogeneous equilibrium

Equilibrium involving the reactants and products in different physical states(phases) is called heterogeneous equilibrium;

$\mathrm{Ex} ; {\mathrm{CaCO}}_3\left(\mathrm{s}\right)\rightleftharpoons \mathrm{CaO}\left(\mathrm{s}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right) ;$