A sheet of BCC iron 4.4 mm thick was exposed to a carburizing atmosphere on one side and a decarburizing 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 0.011 and 0.0070 wt%. Calculate the diffusion coefficient if the diffusion flux is 1.9 × 10-8 kg/m2·s, given that the densities of carbon and iron are 2.25 and 7.87 g/cm3, respectively.