What Causes the Velocity of a P-Wave to Speed Up

Multiple Choice

What causes the velocity of a P-wave to speed up when it enters the inner core?

A) The inner core is a much more dense material than the outer core.
B) The inner core is a much less dense material than the outer core.
C) The inner core is liquid,and thus the wave speeds up.
D) The P-wave no longer has to compete with the S-wave for energy.