Which Effects Have Been Useful (And Successful)in the Search for and Identification

Multiple Choice

Which effects have been useful (and successful) in the search for and identification of black holes in the universe?

A) the influence of their intense gravitational field on atoms that are emitting light from the event horizons of the black holes
B) their gravitational influence on nearby matter, particularly companion stars
C) the effect of their angular momentum, or spin, on nearby matter
D) their magnetic fields and the influence of these fields on nearby matter