[单选题]
In seismology, the Srwy jjy8:. *mw wave is a h .h+gapmbzqod*,, g8transverse wave. As an S wave travels through the Earth, the relative motion between the S wave andoghhdm 8b aq*g,,z +.p the particles is
A. parallel.
B. perpendicular.
C. first parallel, then perpendicular.
D. first perpendicular, then parallel.
