9. In graph 2 both PPFs intersect the houses axis at the same point, but intersect the wheat axis at different points. If all resources are being used to produce wheat, more can be produced on one PPF than on the other, but that isn't true for houses. Compare this with 3 where more of both houses and wheat can be produced on one PPF than on the other, or with 1 where more houses but not more wheat can be produced.
