What Observational Evidence Seems to Indicate the Presence of a Supermassive

What observational evidence seems to indicate the presence of a supermassive black hole at the center of the Milky Way's neighboring galaxy, M31, in Andromeda?

A) slow but measurable motion of the Milky Way Galaxy toward M31 under the intense gravitational force of its black hole
B) very intense X-ray emission from a very small central core, indicating a very hot source
C) spectroscopic measurements of extremely high and symmetric Doppler shifts from stars orbiting rapidly around a massive object near the galactic center
D) small but dark region on detailed photographs of M31, showing an area from which light cannot escape