Assume that Mark and Maria always consume a positive amount of each commodity, and they both have conventional convex-to-the origin indifference curves. Mutually beneficial exchange can necessarily occur as long as

(a) Mark and Maria have different marginal rates of substitution between commodities. (b) Mark and Maria are at a point on the contract curve. (c) The distribution of all goods between Mark and Maria is unequal. (d) Both a and b. (e) Both a and c. (f ) Both b and c.