RR produces and sells industrial sized air cleaners in a plant located in the Pacific Northwest of the United States. Each finished unit includes two specialized designed filters. These filters represent a significant part of the cost of the air cleaners. 18. The cost of the filters. a. Direct material b. Direct labor C. Indirect material d. Indirect labor e. Other manufacturing overhead 19. Salary paid to the shift supervisor in the plant. a. Direct material b. Direct labor C. Indirect material d. Indirect labor e. Other manufacturing overhead