The paragraph is primarily concerned with A. explaining how spiral-arm patterns are formed in large galaxies. B. explaining why large and dwarf galaxies differ in density C. suggesting that the shape of a galaxy is likely to have as great an effect on the number of stars it forms as does its chemical makeup. D. providing information that might be useful in understanding differences in the pace at which stars are formed in different galaxies. E. relating discoveries made about differences in the gravitational forces that exist among galaxies.