Which of the following costs is classified as a period cost under variable costing? O a. Fixed manufacturing overhead O b. Variable manufacturing overhead O c. The cost of direct materials O d. The cost of direct labor Which of the following is true of a fixed cost? O a. Fixed costs in total vary in direct proportion to changes in output within the relevant range. O b. The per unit fixed cost increases with an increase in the level of output. O c. Fixed costs in total are constant within the relevant range as the level of output changes. Od. The per-unit fixed cost is always constant irrespective of the number of units produced