The X-Ray Source Cygnus X-1 Is a Black Hole Candidate

Multiple Choice

The X-ray source Cygnus X-1 is a black hole candidate located in a binary star system. The X-ray source is believed to occupy a volume smaller than Earth. This size is deduced from

A) rapid flickering in the X-ray brightness of Cygnus X-1.
B) Cygnus X-1's apparent magnitude and distance.
C) Cygnus X-1's luminosity and spectral class.
D) the shortness of Cygnus X-1's orbital period.