Cost Classification

Refer to Ch.4 of the course note in Opentution test Q.5 (p.19) Up to a given level of activity in each period the purchase price per unit of a raw material is constant. After that point a lower price per unit applies both to further units purchased and also retrospectively too all units already purchased. Which of the following graphs depicts the total cost of the raw materials for a period? (As the question contains graphs, it would be great if you can refer to F2 course note p.19) I would like to know why the answer is B instead of A