A simple way to model air pollution over a city is with a box model that assumes complete mixing and limited ability for the pollutant to disperse horizontally or vertically except in the direction of the prevailing winds. Consider a town having an inversion at 250 m, a 20-km horizontal distance perpendicular to the wind, a windspeed of 2 m/s, and a carbon monoxide emission rate of 60 kg/s. Assume CO is conseravtive and completely mixed in box. What should be the CO concentration in the box?