Figure 21-13 A graph of y versus x shows 3 straight lines, for convenience called Line 1, Line 2, and Line 3, from bottom to top. Line 1 decreases linearly from (0, 4) to (4, 0). Line 2 decreases linearly from (0, 8) to (2, 0). Line 3 decreases linearly from (0, 8) to (8, 0). The graph also shows 2indifference curves, I C 1, and I C 2, from left to right. The point D, C, and E are listed as follows, from left to right. Point D is at (1, 4) and is tangent at Line 2 and I C 1. Point C is at (2, 2) and is tangent at Line 2 and I C 1. Point E is at (4, 4) and is tangent at Line 3 and I C 2. Points D and E have the same quantity of y but E has 3 more units of x. Refer to Figure 21-13. When the price of X is $80, the price of Y is $20, and the consumer's income is $160, the consumer's optimal choice is D. Then the price of X decreases to $20. The substitution effect can be illustrated as the movement from