The true statement about cost behavior is that:
a. variable costs change on a per-unit basis and change in total as activity changes.
b. fixed costs are constant on a per-unit basis and change in total as activity changes.
c. fixed costs are constant on a per-unit basis and constant in total as activity changes.
d. fixed costs change on a per-unit basis and are constant in total as activity changes.
e. variable costs are constant on a per-unit basis and are constant in total as activity changes.