B company, a text book manufacturer, incurred the following costs for the last 3 months: Production in units: 40,000 50,000 60,000 Factory rent: $10,000 $10,000 $10,000 Indirect Manufacturing costs: $50,000 $60,000 $70,000 Selling costs: $60,000 $75,000 $90,000 Commissions: $120,000 $150,000 $180,000 Which of the following costs is variable? Select one: a. Both Commissions and Selling Costs b. Commissions c. Factory Rent d. Selling Costs