[单选题]
In seismology, the P wave is a longitudinal wave.airp5l cje/sq3,7p:v As a P wave travels 7czgdsge5o3dd -3 )asthrough the Egd-d )z3 ad5s esgoc73arth, the relative motion between the P wave and the particles is
A. parallel.
B. perpendicular.
C. first parallel, then perpendicular.
D. first perpendicular, then parallel.
