Why does the professor discuss a series of alternating bright and dark ring around a star? A. To point out a limitation of the traditional coronagraph. B. To explain how the color of the coronagraph's disk is important to the coronagraph effectiveness. C. To explain a problem with the technology of the vortex coronagraph that has not been solved. D. To emphasize the importance of using a lens that is just the night size for the telescope.