The Possible Presence of a Supermassive Black Hole at the Center

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

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