Which of these is not an argument for Cygnus X-1 being a black hole?
A) The mass of the X-ray source is about 10 solar masses.
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) All of these are arguments.
