1. Economists use indifference curves to characterize the preferences of a rational consumer. There are some plausible assumptions that lead to a set of "well-behaved" (i.e., downward-sloping and strictly convex) indifference curves. Which one of the following is NOT among the assumptions? a. All goods must be economic "goods." b. Preferences are complete. c. Preferences are transitive. d. All goods are normal goods