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 of about 5 solar masses, such as a black hole?

A) infrared imaging of a region whose effective temperature is lower than the cosmic microwave background, rendering it dark
B) measurement of the gravitational redshift of spectral lines in the spectrum of the object
C) measurement of the effect of its gravitational force on a companion object in a binary system
D) estimation of the luminosity of the object and the application of the mass-luminosity relationship