The Diameter of the Black Hole Candidate Cygnus X-1 Has

The diameter of the black hole candidate Cygnus X-1 has been estimated by

A) combining the amount of energy received at Earth per second with the distance to the source to estimate the total energy emitted per second.
B) measuring how quickly it flickers in intensity.
C) measuring how long it takes to block off the light from its companion star during an eclipse (i.e. ,as it passes in front of its companion star as seen from Earth) .
D) combining its distance with its angular diameter to find its physical size.