variable costs

Which of the following statements describes variable costs?
A. Costs that vary on a per-unit basis as the level of activity changes.
B. Costs that vary in total in direct proportion to changes in the level of activity.
C. Costs that remain the same in total dollar amount as the level of activity changes.
D. Costs that vary on a per-unit basis, but remain the same in total as the level of activity changes.