Water enters a lake at the rate of 2 cubic kilometers per day. Water evaporates from the lake at the rate of 1.2 cubic kilometers per day, and the dam at the end of the lake regulates the flow so that the volume in the lake stays at a constant 245 cubic kilometers. The water entering the lake contains 2.9 tons of salt per cubic kilometer, and the water leaving the lake is well mixed. The differential equation for the amount of salt in the lake, measured in tons, with time measured in days, is. a) Q' = 8.41 - 0.00 163 Q b) Q' = 8.7 - 0.00816 Qc) Q' = 4.2 - 0.0049 Qd) Q' = 5.8 -0.00327 Q