At an Age of 380,000 Years, the Temperature of the Universe

Multiple Choice

At an age of 380,000 years, the temperature of the universe had fallen to 3000 K, and electrons could then combine with protons to produce neutral hydrogen gas. What major transition took place in the universe at this time?

A) The present laws of physics began to apply for the first time.
B) The universe suddenly lost its electrical charge and became neutral.
C) Nuclear fusion no longer occurred below this temperature.
D) The universe became transparent to light for the first time.