Which of the following is the best answer to the question "Why does the Sun shine?"
A) The Sun initially began generating energy through nuclear fusion as it formed, but today, it generates energy primarily through the sunspot cycle.
B) The young Sun became hotter through gravitational contraction. Once the core was hot enough, nuclear fusion began and has kept the Sun shining ever since.
C) The Sun initially began making energy through chemical reactions. These heated the interior enough to allow gravitational contraction and nuclear fusion to occur.
D) As the Sun was forming, nuclear fusion reactions in the shrinking clouds of gas slowly became stronger and stronger, until the Sun reached its current luminosity.
B) The young Sun became hotter through gravitational contraction. Once the core was hot enough, nuclear fusion began and has kept the Sun shining ever since.
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