Calculate the mass flux of carbon under steady-state diffusion through a steel plate separating carbon-rich and carbon-poor gases at 700 °C if subsequent analysis gives carbon concentrations of 1 kg/m³ and 0.5 kg/m³ at the respective depths of 0.5 mm and 1 mm below the surface. Assume 10⁻¹¹ m²/s as the diffusion coefficient of C in the iron matrix at this temperature.