Which Kind of Stars Are the Major Source of Energy

Multiple Choice

Which kind of stars are the major source of energy for the heating of the dust clouds and the H II emission nebulae within the planes of the Milky Way and other galaxies?

A) Hot,young O and B stars,via their UV radiation
B) The numerous old,red-giant K and M stars,via their IR heat radiation
C) Very hot white dwarf stars,the remnants of planetary nebulae in the gas clouds
D) The very many nova and supernova explosions of stars within the gas and dust clouds