Planetary nebulae are so-named because A) these extended objects, often green-colored, looked like planets when first seen by nineteenth-century observers through their telescopes. C) the ejected material is rich in carbon and oxygen, necessary elements for the manufacture of planets in the nebulae surrounding stars. C) they rotate slowly and condense into planetary objects around a central star. D) their spectra appear to be similar to the spectrum of the giant gas planets in our own solar system.