The cost of indirect materials used in production is credited to the Raw Materials Inventory account. Work-in-Process Inventory account. Finished Goods Inventory account. Manufacturing Overhead account. Variable costs are fixed per unit and vary in total as production levels change. vary per unit of output as production levels change. are fixed in total as production levels change. decrease per unit as production volume increases. Testing ways to increase the strength of your product would be classified as which element of the value chain? Production Research and development Design Distribution