20-cm long open-ended, horizontal pipe stub on the sample port of a flare line is initially filled with methane when sampling is completed and the sample system is removed. Assume the pipe tube can be modeled as a horizontal tube that is initially completely filled with methane up to the exit plane, beyond which is pure air. Assume the diffusivity of methane in air is 0.000021 m2/s. a) Write a governing differential equation describing the diffusion of air into the pipe stub. What assumptions are made to describe the equation.