Figure10A-10Consumer Choice Between Perfect Substitutes When two goods are perfect substitutes, small price changes can lead to large changes in the consumption bundle. In panel (a), the relative price of chocolate chip cookies is slightly higher than the marginal rate of substitution of chocolate chip cookies in place of peanut butter cookies; this is enough to induce Cokie to choose consumption bundle A, which consists entirely of peanut butter cookies. In panel (b), the relative price of chocolate chip cookies is slightly lower than the marginal rate of substitution of chocolate chip cookies in place of peanut butter cookies; this induces Cokie to choose bundle B, consisting entirely of chocolate chip cookies.