Which statement is true for particles of the medium of an earthquake p-wave? a. they vibrate in a direction parallel to that of the wave, forming crests and troughs. b. they vibrate in a direction parallel to that of the wave, forming compresions and rarefactions. c. they vibrate in a direction perpendicular to that of the wave, forming crests and troughs d. they vibrate in a direction perpendicular to that of the wave, forming compressions and rarefactions