A sheet of BCC iron 1.5-mm thick was exposed to a carburizing atmosphere on one side and an carburizing atmosphere on the other side at 725°C. After having reached steady state, the iron was quickly cooled to room temperature. The carbon concentrations at the two surfaces were determined to be 0.012 and 0.0069 wt%. Calculate the diffusion coefficient if the diffusion flux is 3.6 × 10-8 kg/m2-s, given that the densities of carbon and iron are 2.25 and 7.87 g/cm3, respectively.