Two costs at Bradshaw Company appear below for specific months of operation. Month Amount Units Produced Delivery costs September $40,000 40,000 October 55,000 60,000 Utilities September $84,000 40,000 October 126,000 60,000 Which type of costs are these? (a) Delivery costs are mixed and utilities are variable. (b) Delivery costs and utilities are both variable. (c) Delivery costs and utilities are both mixed. (d) Utilities are mixed and delivery costs are variable