It Was Originally Believed That in the Seismic Shadow Zone

Multiple Choice

It was originally believed that in the seismic Shadow Zone no seismic waves could be detected at all.But as detection techniques improved,it was realized that faint P waves could be detected there.To what do we attribute the source of these P waves?

A) Slight earthquakes deep within Earth
B) The existence of an almost impenetrable layer of solid iron surrounding the outer core
C) Meteor impacts on the side of Earth away from the Shadow Zone
D) Reflections from a solid inner core