21. The marginal cost of a product can be thought of as the cost of producing one additional unit of output. For example, if the marginal cost of producing the 50th product is $6.20, it cost $6.20 to increase production from 49 to 50 units of output. Suppose the marginal cost C (in dollars) to produce x thousand mp3 players is given by the function

C(x) = x² - 120x+7100.

A. How many players should be produced to minimize the marginal cost?

B. What is the minimum marginal cost?