please explain

Consider each of the following systems at equilibrium. The temperature is kept constant, and in each case the reactants are in a cylinder fitted with a movable piston. For which system(s) will a decrease in pressure favor the formation of products? Check all that apply. \begin{tabular}{|l|} A(s)2B(s) \\ 2A(g)2B(g) \\ 2A(g)B(g) \\ \hline \end{tabular} None of the above