After the Big Bang, in Order for the Universe to Become

Multiple Choice

After the Big Bang, in order for the universe to become transparent to light and other electro-magnetic radiation, what had to happen?

A) stars and galaxies had to form
B) the whole universe had to be hotter than the interior of a star
C) the density of the universe had to decrease (to 1000 nuclei per cubic centimeter or less)
D) the dark energy had to dominate over regular matter and energy
E) telescopes had to be invented