Robert only consumes X and Y, and his indifference curves have the usual convex shape. Consider the consumption bundles (3, 9), (6, 6), and (9, 3) (hint: they lie on a straight line). If Robert is indifferent between (3, 9) and (9, 3), then:

A) he prefers (3, 9) over (6, 6).

B) he prefers (9, 3) over (6, 6).

C) he prefers (6, 6) over both (3, 9) and (9, 3).

D) he prefers (6,6) over (3,9) but not over (9,3).