Select 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 materials cost is $1.80 per pound b. Total cost is fixed at $7,500 per month regardless of the number of units produced. c. Auto rental has a fixed fee of $70.00 per day plus $2.00 per mile driven. d. Machine operating costs include $900 of maintenance per month, and $11.00 of coolant usage costs for each day the machinery is in operation