When did the photons in the cosmic microwave background first travel freely through the universe?
A) the moment of the Big Bang
B) about 10-10 second after the Big Bang
C) about 5 minutes after the Big Bang
D) about 380,000 years after the Big Bang
E) At the time, the first stars began to shine in the universe.
D) about 380,000 years after the Big Bang
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