The function C gives the production cost for a bakery to produce cakes of a certain type, where C(x) is the cost, in dollars, to produce x of the cakes. The function M defined by M(x)=C(x+1)−C(x) gives the marginal cost, in dollars, to produce cake number x+1. Which of the following gives the best estimate for the marginal cost, in dollars, to produce the 40th cake? A: c(39)/(39)
B: c(40)/(40) -c(39)/(39)
C: c'(39)
D: c'(40)-c'(39)