Why are doorknobs placed at the edge of a door rather than in the center

The field lines around one end of a bar magnet are shown below based on the diagram, what can you conclude about the pole of the magnet? a.is a south pole because the field hnes spread out from this end b.a north pole because the field lines spread out from this end c.a nouth pole because the field lines enter the magnet at this end d.is a north pole because the field lines enter the magnet at this end