Robert Only Consumes X and Y, and His Indifference Curves

Multiple Choice

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) .