Planetary Nebulae Are So Named Because

Planetary nebulae are so named because:

A) they were extended objects, often green-colored, that looked like planets when first seen by nineteenth-century observers through their telescopes.
B) 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 spectra of the giant gas planets in the solar system.