What Originally Caused the Sun to Heat Up

Multiple Choice

What originally caused the sun to heat up?

A) Nuclear fusion reactions that occurred during the time when the sun's gases were spread out and beginning to fall together by gravitational attraction.
B) The gases from which it formed had already been heated by the Big Bang.
C) As the sun's gases gathered together by their own gravity, they accelerated inward and collided.
D) Electromagnetic radiation from outside the sun heated the sun's gases as they gathered together.
E) A nearby supernova explosion that occurred during the early stages of the formation of the sun.