Which of the following might cause total variable cost behavior to appear as a curvilinear function? (Select all that apply.)______
a. Renting additional storage warehouse space as output volume increases to higher and higher levels.
b. Adding additional salaried supervisors as output volume increases to higher and higher levels.
c. Paying overtime wages to direct labor workers as output volume increases to higher and higher levels.
d. Receiving price discounts on larger and larger purchases of direct materials as output volume increases to higher and higher levels.