Why Did the Era of Nuclei End When the Universe

Multiple Choice

Why did the era of nuclei end when the universe was about 380,000 years old?

A) All the free particles had combined to form the nuclei of atoms.
B) The universe had expanded and cooled to a temperature of about 3,000 K,cool enough for stable,neutral atoms to form.
C) Neutrinos and electrons were finally able to escape the plasma of the early universe and no longer heated the other particles.
D) Photons were finally able to escape the plasma of the early universe and no longer heated the hydrogen and helium ions.
E) No theory can explain this.