What Evidence Led Scientists to Believe That Waves Travel at a Greater

What evidence led scientists to believe that waves travel at a greater velocity inside Earth?

A) Studies of earthquake signals via sensors located deep inside drill holes prove it.
B) Refraction of energy waves is most efficient at shallower depths.
C) Refraction of energy waves is most efficient along short paths.
D) Average velocity is faster when the seismograph is farther from the focus of the earthquake.