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Write the mass balance equation for an ideal 1D plug-flow stream under steady state conditions with a first-order decaying substance. Solve for the concentration as a function of the concentration x=0, the rate constant kd, longitudinal distance x, and mean velocity u. Assume the rate constant for BOD decay is 0.2 d^-1, the mean velocity of the stream is 0.5 m s^-1, and the initial concentration is 100 mg L^-1 at x=0. Plot the resulting concentration with downstream distance x in km.