t months after initiating an advertising campaign, s(t) hundred pairs of a product are sold, where S(t) = 3 / t+3 – 13 / (t+3)² + 21. a) Find S' (t) and S" (t) S' (t) = S" (b) At what time will the sales be maximized? What is the maximum level of sales? (c) The program will be discontinued when the sales rate is minimized. When does this occur? What is the sales level at this time? What is the sales rate at this time?