When the Universe Was About 24,000 Years Old, the Radiation $\rightarrow$

When the universe was about 24,000 years old, the radiation density had decreased to the point where it was equal to the matter density, yet it was only many years later that atoms formed. What happened to make possible the formation of atoms?

A) The average photon energy decreased until it could no longer ionize hydrogen.
B) Radiation pressure decreased until it was equal to the inward pressure due to gravity.
C) The temperature reached the point where the reaction ¹H $\rightarrow$ ⁴He could proceed.
D) Neutrons decayed to produce protons and electrons, the building blocks of atoms.