Effects of pressure on equilibrium Which system at equilibrium will not be influenced by a change in pressure?
$$
\begin{align}
\ce{3 O2(g) &amp;&lt;=&gt; 2 O3(g)}\tag{A}\\
\ce{N2(g) + 3 H2(g) &amp;&lt;=&gt; 2 NH3(g)}\tag{B}\\
\ce{2 NO2(g) &amp;&lt;=&gt; N2O4(g)}\tag{C}\\
\ce{H2(g) + I2(g) &amp;&lt;=&gt; 2 HI(g)}\tag{D}\\
\ce{2 W(g) + X(g) &amp;&lt;=&gt; 3 Y(g) + 2 Z(s)}\tag{E}
\end{align}
$$

My attempt: Equilibrium will be influenced if the number of moles on one side of the equation is different than the other side. Thus, I can eliminate option A,B, and C. But how to differentiate between options D and E? They both look right to me, because there are the same amount of moles of gases on both sides, and solids do not play a factor in determining equilibrium.
For reference, the answer given in the book is D.
As Zhe explains in the comments, the answer is D. Even though small changes in pressure are not expected to significantly affect the chemical potential of a solid, it is a function of pressure:

$$\mu(\mathrm{s}) = \mu^\circ(\mathrm{s}) + \int_{p^\circ}^{p}V_\mathrm{m}\,\mathrm dp \qquad T = \mathrm{const}$$
This means that a small change in the chemical potential of the solid also contributes to the total free energy change of the system when the pressure is altered. Usually this response is ignored as $V_\mathrm{m}$ is orders of magnitude greater for gases than for condensed phases.