For the reaction $\mathrm{C}_{\mathrm{ls}}+\mathrm{CO}_2(\mathrm{~g}) \rightleftharpoons 2 \mathrm{CO}(\mathrm{g})$, the partial pressures of $\mathrm{CO}_2$ and CO are 4 and 8 atm respectively. The value of $\mathrm{K}_{\mathrm{p}}$ for this reaction is
(a) 14 atm
(b) 16 atm
(c) 18 atm
(d) 12 atm