The function C gives the cost, in dollars, to produce a particular product, where C(x) is the cost, in dollars, to produce x units of the product. The function M defined by M(x)=C(x+1)−C(x) gives the marginal cost, in dollars, to produce unit number x+1 . Which of the following gives the best estimate for the marginal cost, in dollars, to produce the 57th unit of the product?