Assume that at time t = 0 a tank contains 10 kg of salt dissolved in 90 L of water. Suppose that water containing kg/L of salt is added at a rate of 6L /min and that the well stirred mixture is drained from the tank at the same rate. Let S(t) denote the amount of salt in the tank at time t minutes past 0. Which of the following statements are true? a) S(0) = 10 kg b) S'(0) = 2 kg/min c) lim S(t) = 30 kg t-+ 00 d) 20 kg