Why Do H II Regions Glow

Why do H II regions glow?

A) The hydrogen cannot emit radiation because it is fully ionized, so the radiation comes from neighboring molecules of carbon monoxide.
B) The H II region collides with a giant molecular cloud exciting the molecules in the cloud to radiate.
C) Some H II ions unite with electrons to re-form neutral hydrogen. In this process, the electron passes from level to level in the atom and emits a cascade of photons.
D) Protons, which make up H II, collide with each other, resulting in gamma-ray emission from the nuclei.