After the Big Bang, as the Universe Cooled and Protons

Multiple Choice

After the Big Bang, as the universe cooled and protons and electrons combined so that the universe became neutral, what important consequence happened?

A) Protons and neutrons combined to form nuclei such as deuterium and helium.
B) Neutrinos ceased to interact with normal matter.
C) Dark matter ceased to interact with normal matter.
D) Photons began to travel freely through the universe.
E) Lithium and other light elements were formed by the fusion of hydrogen and helium.