Assume that a unit of output is the cost object. Which of the following statements is valid?

A: Royalties paid on per unit basis are an example of an indirect expense. B: Materials consumed in the maintenance of machinery used to manufacture several different products are an example of a direct material cost. C: The salaries of supervisors who oversee the manufacture of several different products are an example of a direct labour cost. D: Rent paid for a factory in which several different products are produced is an example of an indirect expense.