During an earthquake, primary (P) waves and secondary (S) waves are generated. Which option correctly states the nature of particle motion in each wave?
(a) P-waves: transverse; S-waves: longitudinal
(b) P-waves: longitudinal; S-waves: transverse
(c) P-waves: transverse; S-waves: transverse
(d) P-waves: longitudinal; S-waves: longitudinal