The Equilibrium 3Fe(s)+ C(s) Fe₃C(s)is Established in a Solid

The equilibrium 3Fe(s) + C(s) Fe₃C(s) is established in a solid solution. For such a solution, one can write an equilibrium constant in the usual way except that here one has concentrations that refer to solids in the solid solution. Determine the equilibrium constant for the formation of cementite from iron and carbon at 680°C.(Given: for this reaction at 25°C, $\Delta$ H° = 21 kJ/mol and $\Delta$ S° = 20.4 J/mol·K)

A) 0.75
B) 0.33
C) 3.1
D) 0.82
E) 1.2