Frannie Spends Her Income on Rice and Beans

Frannie spends her income on rice and beans. At her optimum, Frannie's

A) utility from consuming rice is equal to her utility from consuming beams.
B) marginal utility of rice is equal to her marginal utility of beans.
C) marginal utility per dollar spent on rice equals her marginal utility per dollar spent on beans.
D) marginal rate of substitution is equal to 1.