The Vast Majority of the Stars in the Galactic Halo

The vast majority of the stars in the galactic halo are isolated field stars. What is the origin of these stars?

A) These very old stars are left over from the original infall of the material that formed the Galaxy.
B) These are stars left over from the formation of the globular clusters.
C) These are stars that were moving too fast and were expelled from globular clusters.
D) These are the remnants of small galaxies that have been absorbed by the Milky Way Galaxy.