Correct Response: A. There is observational evidence that mergers and collisions of galaxies played a major role in the formation of different types of galaxies in the early universe and that these types of gravitational interactions have continued, especially in galaxy clusters. The composition and structure of different types of galaxies indicate that the structure of large spiral galaxies, elliptical galaxies, and some irregular galaxies results from gravitational interactions during mergers and collisions. Black holes (B) at the center of galaxies are not believed to determine whether a galaxy has a spiral or elliptical structure. Gravitational attraction of the matter in a galaxy is central to that galaxy's evolution (C), but does not explain the origin of different types of galaxies. The relative amount of heavy elements in a galaxy (D) relates to the evolution of the stars in the galaxy, but is not responsible for the evolution of its large-scale structure.
