Which of These Is NOT an Argument for Cygnus X-

Which of these is NOT an argument for Cygnus X- 1 being a black hole?

A) It is the third strongest source of X- rays in the sky.
B) The X- rays from the compact source vary in as little as a millisecond.
C) Spectroscopic data suggests hot gas is flowing toward the X- ray source.
D) The mass of the visible star is greater than that of the X- ray source.
E) The mass of the X- ray source is about 10 solar masses.