Uranium,thorium,and the Radioactive Isotope of Potassium Cannot Supply the Heat

Uranium,thorium,and the radioactive isotope of potassium cannot supply the heat to melt small planetesimals,yet are sufficient to melt the interior of Earth.This is because these elements

A) decay slowly,and the heat generated must be trapped by a thick crust if it is to melt the interior.
B) were not present in the solar nebula at the location where the asteroids formed.
C) are only found in the outer layers of asteroids and not in the core.
D) are very volatile and evaporated from the asteroids before they could decay.
E) are only found on rocks that formed on the earth and in no other celestial objects.