The Inner Core Is Believed to Rotate Independently from the Outer

The inner core is believed to rotate independently from the outer layers of Earth.Why is this possible?

A) The immense interior pressures generate rotation.
B) The inner core is separated from the mantle by the liquid outer core and can spin freely.
C) Radioactive decay generates spin.
D) Frictional drag pushes the inner core at a different rate than the rest of the planet.