Supermassive black holes are believed to be located at the center of many galaxies because a. the rotation curve of the galaxy indicates that 90% of the galaxy is dark matter. b. the orbital speeds of a globular clusters in the galaxy are greater than the speed of light. c. the orbital motion of material near the center is very fast and indicates a very massive core. d. the shape of the bulge in all spiral galaxies can only be supported by a supermassive black hole. e. the spiral structure requires a black hole to maintain the spiral arms.