An economy can produce 80 units of good X and 4 units of good Y or 70 units of good X and 6 units of good Y by fully and efficiently using available resources. If the opportunity cost of producing good Y (relative to good X) is increasing, which of the following combinations of goods X and Y can be a point on the production possibilities curve for that economy? a) 73 X and 5Y; b) 76 X and 5 Y; c) 74 X and 5 Y; d) 17 X and 5 Y.

Why is b) the correct answer?