What Evidence Now Exists for a Supermassive Black Hole at the Center

What evidence now exists for a supermassive black hole at the center of the Milky Way Galaxy?

A) very bright, steady X-ray emissions from the galactic center
B) observations of intense inflow of matter toward the center of the Galaxy as seen by light, Doppler-shifted toward the red, emitted by this matter
C) very rapid motion of matter close to the nucleus of the Galaxy, requiring a very massive body to hold it in orbit
D) very dark void in an otherwise bright region of space near the galactic center, indicating the presence of a black hole