The difference between variable costs and fixed costs is :

(A) Variable costs per unit fluctuate and fixed costs per unit remain constant

(B) Variable costs per unit are fixed over the relevant range and fixed costs per unit are variable

(C) Total variable costs are variable over the relevant range and fixed in the long-term, while fixed costs never change

(D) Variable costs per unit change in varying increments, while fixed costs per unit change in equal increments