What Is the Evidence That Indicates to Some Astronomers That

What is the evidence that indicates to some astronomers that a supermassive black hole exists at the center of the Milky Way Galaxy?

A) A Doppler shift of light from stars in the near neighborhood of the galactic center indicates that the stars are falling inward at very high speeds.
B) The Sun's motion in space shows that, if Kepler's law holds for its orbit around the galactic center, there must be a very massive object at the center of the Milky Way.
C) No electromagnetic radiation at all comes from the precise position of the galactic center, and it just looks like a dark void in space.
D) Measurement of gas clouds orbiting the galactic center at very high speeds indicates that they would rapidly move out of the Galaxy unless some very massive object were holding them in orbit.