[单选题]
In seismology, the P wave is a longitudin7 ftca/9a2y,e q 4jniwal wave. As a P wave travels througo;c 1 x9kmz9vvh the Earth, the relative motion between the P wave and the particlo;1k mv9vzc9 xes is
A. parallel.
B. perpendicular.
C. first parallel, then perpendicular.
D. first perpendicular, then parallel.
