What Method Is Used by Astronomers to Infer the Existence

Multiple Choice

What method is used by astronomers to infer the existence in space of a dark object with a mass of about 5 solar masses such as a black hole?

A) The measurement of the effect of its gravitational force on a companion object in a binary system
B) The measurement of the gravitational redshift of spectral lines in the spectrum of the object
C) The estimation of the luminosity of the object and the application of the mass-luminosity relationship
D) The photography or imaging of a region from which no light or radiation at all appears to come