Malik, the business manager of a company that produces in-ground outdoor spas, determines that the cost of producing x spas is C thousand dollars, where

C(x) = 0.04x² + 2.1x + 60

a. If Malik decides to increase the level of production from x = 10 to x = 11 spas, what is the corresponding average rate of change of cost?

b. Next, Malik computes the instantaneous rate of change of cost with respect to the level of production x. What is this rate when x = 10, and how does the instantaneous rate compare with the average rate found in part (a)? By raising the level of production from x = 10, should Malik expect the cost to increase or decrease?