As m is a constant that is applied to x, it has to have something to do with minutes, eliminating A and B. As the constant is 2, it has to have something to with 2, eliminating D, meaning your answer is C. This is because a slope is the rate of change of y in relation to x, so it would be rate of change of y is a (a being your constant, in this case 2) per x. With your case, y is cost and x is minutes, leading to answer choice C.