We Find That Interstellar Gas and the Youngest Disk Stars

We find that interstellar gas and the youngest disk stars are found in a very narrow distribution along the galactic plane, but that older disk stars can be found in a thicker disk. What might this mean?

A) Gravitational scattering off molecular clouds changes the orbits of disk stars as they get older.
B) The disk is still settling down from a thicker to a thinner state.
C) The galaxy has not absorbed any dwarf galaxies in the last several billion years.
D) Cosmic rays are responsible for causing star formation to happen in a narrow layer over time.
E) Most of the dark matter in the galaxy is in a thin layer along the disk.