What Evidence Is There for Considerable Extra Mass Within Galaxies

Multiple Choice

What evidence is there for considerable extra mass within galaxies that does not produce visible light,the so-called "dark matter?"

A) The rotation curve of galaxies,showing orbital speeds of material,remains flat to large distances from the galactic centers and does not follow a Keplerian curve.
B) The appearance of many very dark spots within the galaxy,evidence of numerous low-mass black holes scattered throughout the galaxy
C) Widespread evidence for gravitational lensing of background stars in a galaxy by massive,but invisible,objects in the foreground
D) Intense output of X-rays from very hot gas between the stars,originating in otherwise dark regions