The diagram below that accompanies this question illustrates curves for a firm operating in a differentiated-product oligopoly. One of the demand curves is relevant when rivals match the firm's price changes; the other demand curve is relevant when rivals do not match price changes.

Which answer is most relevant when rivals will match any price change?

a) Demand A & MR-A

b) Marginal Cost

c) Demand A & MR-B

d) Demand B & MR-B