Figure 21-9 Graph (a) Graph (b) Graph (c) A graph of y versus x shows two indifference curves, descending linearly parallel to each other. A graph of y versus x shows two 90 degree angled lines that are indifference curves and are parallel to each other. A graph of y versus x shows two indifference curves, decreasing from left to right, that are parallel to each other. The left curved line is slightly more curved than the right curved line. Refer to Figure 21-9 . Which of the graphs illustrates indifference curves for which the marginal rate of substitution is constant? a. Graph (a) only b. All of the graphs show a constant marginal rate of substitution. c. Graph (c) only