Abe starts exercising regularly, and after a few months he can do twice as much of everything. in a single day, abe can now make 10 hamburgers or 8 milkshakes, rather than the 5 hamburgers and 4 milkshakes he made in the past. we now know that abe's production possibility frontier has but his opportunity costs of making milkshakes have shifted right; not changed shifted right; decreased not changed; increased not changed; decreased