Earth's Interior Received Its Heat Energy from the Impacts of Planetesimals

Earth's interior received its heat energy from the impacts of planetesimals and from radioactivity. This heat energy is gradually radiating into space. As Earth's interior continues to cool, we can expect that, in the distant future, the:

A) inner core will become molten like the outer core.
B) outer core will become solid like the inner core.
C) mantle will become molten while the core will become solid.
D) entire interior will become one homogeneous solid.