F2 Specimen Exam applicable from June 2014

Hi

The question :

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 to all units already purchased.
Which of the following graphs depicts the total cost of the raw materials for a period?

The answer is D. Why is A not applicable? After a certain point, both graphs’ further purchases, slopes at lower gradients ..

Please help.