[单选题]
In seismology, the S wave is a trans7 k4ochqnu+v 1verse wave.jcp,ff s(h+ *n As an S wave travels through the Earth, the relafj+(n,p* cfhstive motion between the S wave and the particles is
A. parallel.
B. perpendicular.
C. first parallel, then perpendicular.
D. first perpendicular, then parallel.