Janitors clean the factory with scrubbing machines and polishing machines. Scrubbing machines scrub the factory floor and polishing machines polish the floor. The cost associated with cleaning the factory is treated as a product cost. What is a good cost driver for the Depreciation Expense associated with the scrubbing and polishing machines?

		number of janitors operating machines
		number of kilowatt hours used
		number of labor hours put in by janitors
		number of machine hours used