What Mechanism in the Vicinity of a Star Gives Astronomers

What mechanism in the vicinity of a star gives astronomers a hint of the presence of a black hole as a companion to the star?

A) The light from the companion star shows extreme redshift because of the gravitational field of the black hole.
B) The star periodically disappears from the viewpoint of Earth during its eclipses by the black hole as the two objects orbit each other.
C) The space near the star darkens, indicating that the black hole prevents the light from distant objects from reaching Earth.
D) Gas from the star, falling in toward a black hole, is compressed to very high densities and temperatures so that it emits an intense and rapidly fluctuating flux of X-rays.