What Key Event Happened During the Decoupling Epoch

Multiple Choice

What key event happened during the decoupling epoch?

A) Pairs of neutrons and protons were created.
B) Electrons and positrons were created.
C) Expansion cooled the universe enough that protons could capture electrons in orbit.
D) Dark energy accelerated the cosmos on to infinity.
E) The universe underwent a brief period of very rapid expansion.