Which one of the following pairs is an example of polar molecular solids?
\( \mathrm{SO_2(s)}, \ \mathrm{CO_2(s)} \)
\( \mathrm{MgO(s)}, \ \mathrm{SO_2(s)} \)
\( \mathrm{SO_2(s)}, \ \mathrm{NH_3(s)} \)
\( \mathrm{HCl(s)}, \ \mathrm{AlN(s)} \)