How Has the Mass of the Black Hole Candidate Cygnus

Multiple Choice

How has the mass of the black hole candidate Cygnus X-1 been estimated?

A) from the periodic wobble it produces in the spectral lines of a normal companion star around which it orbits
B) from the gravitational redshift, where the more massive the object, the greater the redshift of its spectral lines
C) from the observed size and estimated density of the object
D) from the periodic wobble in its own characteristic black-hole spectrum