Earth's Innermost Core Is Solid, Not Liquid, Because

Multiple Choice

Earth's innermost core is solid, not liquid, because

A) the core temperature is too low to melt iron.
B) differentiation caused all of the heavy, solid material to sink to the bottom while Earth was forming.
C) all the liquid has moved up into the mantle via convection.
D) the pressure is too high for the material to be in a liquid state.
E) iron does not melt.