Figure 21-15 on the Graph, Q_x Represents the Quantity of Good

Figure 21-15 On the graph, Q_x represents the quantity of good x and Q_y represents the quantity of good y. The lines drawn on the graph represent three of Barbara's indifference curves.
-Refer to Figure 21-15. For Barbara, the marginal rate of substitution between goods y and x

A) increases as she moves downward and to the right along one of her indifference curves.
B) decreases as she moves downward and to the right along one of her indifference curves.
C) remains constant as she moves downward and to the right along one of her indifference curves.
D) is undefined for any movement along any one of her indifference curves.