During formation, the Sun evolved toward the main sequence as shown in the figure. The Sun will evolve away from the main sequence when
a) its core begins fusing iron.
b) its supply of hydrogen is used up.
c) the carbon core detonates, and it explodes as a Type I supernova.
d) helium builds up in the core, while the hydrogen-burning shell expands.
e) the core loses all of its neutrinos, so all fusion ceases
Answer: d) helium builds up in the core, while the hydrogen-burning shell expands.