(Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 15 to 19.

Going Inside Black Holes

     One of the strangest phenomena in the universe is the black hole. For years, scientists have studied black holes in an (15) ______ to better understand how they function. Like vacuum cleaners, black holes will suck up anything (16) ______ crosses their path. The incredible sucking power that black holes generate comes from gravity. They can quickly swallow up anything including planets, space debris, and anything else imaginable. Even light cannot escape the (17) ______ of black holes. Since they are able to pull in light, black holes are nearly impossible to see even with high-powered telescopes. (18) ______ scientists are able to detect the presence of black holes in space because of their effect on an observed area. 

     Black holes can originate in a few ways. One type of black holes occurs when a star comes to the end of its lifecycle and it dies in a supernova explosion. They can also occur when the mass of a neutron star becomes so great that it collapses in on itself. Black holes may also occur when several large and dense stars collide with one (19) ______ in space.