Cost-Volume-Profit Analysis Is Based on Three Basic Assumptions

Cost-volume-profit analysis is based on three basic assumptions. Which of the following is not one of these assumptions?

A) Total fixed costs remain constant over changes in volume.
B) Curvilinear costs change proportionately with changes in volume throughout the relevant range.
C) Variable costs per unit of output remain constant as volume changes.
D) Sales price per unit remains constant as volume changes.
E) All of these are basic assumptions.