The term "variable costs" refer to:

a. all costs that do not change in total for a given period and relevant range, but become progressively smaller on a per unit basis as volume increases

b. all costs that are associated with marketing, shipping, warehousing, and billing activities

c. all costs whose total amounts change in proportion to changes in activity within a relevant range

d. all costs that are likely to respond to the amount of attention devoted to them by a specified manager

e. all manufacturing costs incurred to produce units of output