The Possible Presence of a Supermassive Black Hole at the Center

The possible presence of a supermassive black hole at the center of the Milky Way Galaxy has been deduced from:

A) gravitational radiation emitted by stars as they are swallowed by the black hole.
B) powerful magnetic fields in the huge filaments arching away from (or toward) the center.
C) the number of globular clusters that concentrate toward the galactic center.
D) the very high orbital speed of ionized gas clouds close to the galactic center.