Which of the following best describes the behavior of a variable cost? a. The company itself decides how it will define the behavior of the variable cost. b. In total, it varies directly and proportionally with changes in activity level, but per constant unit, within the relevant range of activity. C. It is associated with the cost of rent, depreciation, property taxes. d. In total it remains constant but unitarily it varies directly and proportionally with the changes in activity level, within the pertinent range of activity. it keeps