When a Solar-Mass Star Is Ejecting a Planetary Nebula,the Temperature

When a solar-mass star is ejecting a planetary nebula,the temperature of its exposed core can reach 100,000 K.How does this temperature compare with the core temperature when the star is on the main sequence?

A) The exposed core is cooler simply because it is exposed to the cold regions of space.
B) The exposed core is cooler because it is at the end of its fuel supply.
C) The exposed core is hotter because it was heated by helium fusion rather than hydrogen fusion.
D) The exposed core is hotter because it was heated by carbon-oxygen fusion rather than hydrogen fusion.