A Planetary Nebula Is

A planetary nebula is:

A) a contracting spherical cloud of gas surrounding a newly formed star, in which planets are forming.
B) the expanding nebula formed by the supernova explosion of a massive star.
C) an expanding gas shell surrounding a hot, white dwarf star.
D) a disk-shaped nebula of dust and gas from which planets will eventually form, easily photographed around relatively young stars.