As a Main-Sequence Star Burns Its Core Supply of Hydrogen

As a main-sequence star burns its core supply of hydrogen, what happens?

A) Helium begins to fuse throughout the core.
B) Helium fuses in a shell surrounding the core.
C) Helium fusion takes place only at the very center of the core, where temperature and pressure are highest.
D) Helium builds up in the core.
E) Helium builds up everywhere in the star's interior.