Let Y = total costs, X_1 = production volume, and X_2 = number of setups. Which of the following are linear cost functions? Which are mixed cost functions? a. Y = $8X_1 b. Y = $1, 500 c. Y = $8, 500 + $1.50X_1 d. Y = $3,000 + $6X_1 + $30X_2 e. Y = $9,000 + $3(X_1 times X_2) f. Y = $5,000 + $4.00X_1