Question

To obtain evidence for the presence of black holes, scientists must

a. use bigger telescopes

b. look for the effect of black holes on surrounding stars

c. look for trails left by black holes as they move through space

d. look for stars giving off black light

Answer 1

i think look for the effect of black holes on surrounding stars.

Answer 2

Black holes likely also reveal their presence by generating powerful bursts of gamma rays .Also like X rays, gamma rays captured the attention of astronomers in the 1960s . hence A is the answer
for more reading http://www.scienceclarified.com/scitech/Black-Holes/Detecting-Black-Holes-Through-Indirect-Means.html