Consider the cost function C(x) = 5x + 20x + 21 (thousand dollars). a) What is the marginal cost at production level x = 4? b) Use the marginal cost at x = 4 to estimate the cost of producing 4.25 units. c) Let R(x) = - x6 + 33x + 36 denote the revenue in thousands of dollars generated from the production of x units. What is the break-even point? (Recall that the break-even point is when the revenue is equal to the cost.) d) Compute and compare the marginal revenue and marginal cost at the break-even point. Should the company increase production beyond the break-even point? a) The marginal cost at production level x = 4 is $