9. The following data pertain to activity and costs for two months: June July Activity level in units 10,000 20,000 Variable costs $20,000 ? Fixed costs 15,000 ? Mixed costs 10,000 Total costs $45,000 $90,000 Assuming that these activity levels are within the relevant range, what were the mixed co for July? a. $15,000. b. $10,000. c. $40,000. d. $35,000