[单选题]
In seismology, the Pqr-2 k*pkv da umwz-0: wab/qx7zrk/ja72 4avyive is a longitudinal wave. As a P wave travels through the Earth, the relative motion between the P wave and the pav4azkxy brq/7 a/2i7j rticles is
A. parallel.
B. perpendicular.
C. first parallel, then perpendicular.
D. first perpendicular, then parallel.
