How Have Astronomers Measured the Mass of the Black Hole

How have astronomers measured the mass of the black hole at the center of our galaxy?

A) by using the rotation curve derived from 21-cm emission
B) by observing the motions of stars near the center of the galaxy
C) by measuring the brightness of the quasar
D) by measuring the Doppler shift of Sagittarius A*
E) by counting the number of supernova explosions near the black hole during the last century