A critical assumption about the behaviour of costs within the relevant range is

(a) variable cost per unit and fixed cost per unit remain constant as sales volume increases.

(b) variable cost per unit decreases and selling price per unit remains constant as sales volume decreases.

(c) variable cost per unit remains constant and fixed cost per unit decreases as sales volume increases.

(d) selling price per unit increases and fixed costs per unit decreases as sales volume decreases.