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 materials cost is \($1.70\) per pound.

b. Total cost is fixed at \($8,000\) per month regardless of the number of units produced.

c. Auto rental has a fixed fee of \($80.00\) per day plus \($2.00\) per mile driven.

d. Machine operating costs include \($1,000\) of maintenance per month, and \($12.00\) of coolant usage costs for each day the machinery is in operation.