The Major Source of Light in the Expanding Shell of Gas

The major source of light in the expanding shell of gas in a planetary nebula is:

A) fluorescence of the atoms, caused by UV light from the hot central white dwarf star.
B) reflection and scattering of the light of the central white dwarf star from dust and gas in the shell.
C) thermal heating of the dust grains by radiation from the hot central star.
D) thermonuclear reactions in this hot gas, caused by the underlying explosion.