(6) A well-mixed lake with a surface area of 3.0 km? and an average depth of 5 m is fed by astream and by rainfall. (Groundwater and runoff are insignificant inputs.) The stream's flow rateis 0.2 m/s, and the concentration of phosphorus in the stream is 0.1 mg/L. Average rainfall is 0.7m/yr, and the rain has a phosphorus concentration of 0.01 mg/L. Phosphorus removal byevaporation and by biological processes is negligible. Phosphorus precipitates from the lake'swater according to first-order kinetics, with a rate constant of 0.005/d. The outlet stream has aflow rate of 0.27 m/s, which keeps the volume of water in the lake essentially constant. Theconcentration of phosphorus in the lake is most nearly (A) 0.018 mg/L (B) 0.029 mg/L (C) 0.034 mg/L (D) 0.045 mg/L

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