For the reaction  ${\mathrm{C}}_{\left(\mathrm{s}\right)}+{\mathrm{CO}}_{2\left(\mathrm{g}\right)}\rightleftharpoons 2{\mathrm{CO}}_{\left(\mathrm{g}\right)}$ The partial pressure of CO₂ and CO are 2.0 and 4.0 atm respectively are equilibrium. The  ${\mathrm{K}}_{\mathrm{p}}$  for the reaction is