Which of the following statements is true? (You may select more than one answer.)

a. The costs of a unit-level activity should be proportional to the number of units produced.

b. Unit-level activities are performed each time a unit is produced.

c. Setting up equipment is an example of a unit-level activity.

d. Activity-based costing systems may use direct labor-hours and/or machine-hours to assign unit-level costs to products.