9. CHEUNG Felix Vitor Which of the following is true? Select one: O a. The relevant range of activity refers to the level of activity where all costs are constant and it is a cost that is incurred by a company that must be accounted for. O b. The relevant range of activity refers to the activity level where all costs are curvilinear and it directly impacts the number of units of product a customer buys. c. The relevant range of activity refers to the geographical areas where the company plans to operate and it is a requirement under GAAP. d. The relevant range of activity refers to the levels of activity over which the company expects to operate and the cost behavior outside of the relevant range is not linear