Just below the point where a continuous discharge of pollution mixes with the river, the BOD is 10.9 mg/L and the DO is 7.6 mg/L. The river and waste mixture has a temperature of 20oC, a deoxygentation constant of 0.2/day, an average velocity of 0.30 m/s and an average depth of 3.0 m. a. Find the time and distance downstream at which the critical point occurs. b. Find the minimum DO value.