The Event in the Life of a Star That Begins

The event in the life of a star that begins its expansion into a giant is

A) the core reaches a temperature of ten million degrees
B) as much as 90% of the star explodes violently
C) almost all the hydrogen in its core that was hot enough for fusion has been turned into helium
D) the star's internal structure reaches equilibrium for the first time in its life
E) it reaches the stage that astronomers call the zero-age main sequence