Astronomers Expect Stars near the Edge of the Galactic Disk

Astronomers expect stars near the edge of the galactic disk to have smaller amounts of heavy elements than other parts of the disk because

A) there is more star formation but fewer supernovae at the edge.
B) there is less star formation and fewer supernovae at the edge.
C) there is less star formation even though there are more supernovae at the edge.
D) there is more star formation and more supernovae at the edge.
E) None of these apply; there is no reason to expect the disk to be different based on location.