Write a linear cost function equation for each of the following conditions. Use y for estimated costs and X for activity of the cost driver. a. Direct manufacturing labor is $10 per hour. b. Direct materia ls cost $9.20 per cubic yard. c. Utilities have a minimum charge of $1,000, plus a charge of $0.05 per kilowatt-hour. d. Machine operating costs include $200,000 of machine depreciation per year, plus $75 of utility costs for each day the machinery is in operation