Why Is the Inner Core Solid Even Though It Is

Why is the inner core solid even though it is hotter than the outer liquid core?

A) This is not unusual: hotter things tend to be more solid.
B) The great pressure at Earth's center forces the atoms in the core together, causing it to solidify.
C) Radioactivity cause liquid materials to become solid.
D) The dynamo effect causes ions in Earth's interior to become solid.