For a certain sporting item that is produced in a factory, we have that the income is given by the function I (x) = x (800-4x), while the cost of production is given by C (x) = 0.4x2 + 8x + 800, where x are the units produced. Production volume for maximum profit [Value 3 points] Select one: a. 100 b. 91 c. 90 d. 101