Consider an organic layer of thickness L on a sealed floor. The upper surface of this organic layer is in contact with the atmosphere, and the oxygen concentration just inside the layer is C_A0. The volumetric flow rate of oxygen used during the biochemical reaction and the diffusion coefficient of D_AB in organic matter are also known. Oxygen utilization depends on the local concentration of oxygen and can be expressed as R_A= -₁.C_A (kmol/s.m³). Write a differential equation that gives the variation of oxygen concentration in a thin layer of organic matter. The string is one-dimensional and flat.