How Do We Know That There Is Much More Mass

Multiple Choice

How do we know that there is much more mass in the halo of our galaxy than in the disk?

A) There are so many globular clusters in the halo that their total mass is greater than the mass of stars in the disk.
B) Stars in the outskirts of the Milky Way orbit the galaxy at much higher speeds than we would expect if all the mass were concentrated in the disk.
C) Although the question of mass in the halo was long mysterious,we now know it exists because we see so many brown dwarfs in the halo.
D) The recent discovery of photinos,combined with theoretical predictions,tells us that there must be a huge mass of photinos in the halo.
E) We don't know that there is more mass in the halo;it is only a guess based on theory.