Why Do All Stars Spend Most of Their Lives on the Main

Why do all stars spend most of their lives on the main sequence?

A) because the neutrinos created inside the Sun do not carry any energy away with them
B) because during this stage the star contracts from enormous size to a relatively small ball; this takes a long time
C) because the fuel for energy production in this stage of the star's life is hydrogen; and that is an element every star has lots and lots of
D) because in this stage, the processes inside the star do not generate any energy; thus the star can continue in this stage indefinitely
E) this is an unsolved problem in astronomy, and is an important project for the world's largest telescopes to work on