One of the key differences between fixed costs and variable costs is: Variable costs per unit change in varying increments while fixed costs per unit change in equal increments over the relevant range of activity. Variable costs per unit fluctuate and fixed costs per unit remain constant over the relevant range of activity. Variable costs per unit are fixed and fixed costs per unit are variable over the relevant range of activity. Variable costs per unit change in equal increments while total fixed costs change in proportion to the level of activity over the company's relevant range Total variable costs are fixed and fixed costs per unit never change over the relevant range of activity