经济学画图题,求教

Draw a diagram for a perfectly competitive industry with firms earning normal profits. All firms in the industry use oil as a key input. Using your diagram, illustrate a reduction in the price of oil. Assume carrying charge being constant.
我觉得其中一个图是MC=MR=AR, 但是另一个图应该怎么话呢，oil 作为input~~ 求教