While researching a topic, a student has taken the following notes: Earthquakes start at a point called a 'focus' and spread out as seismic waves. Two types of seismic waves that travel beneath the surface are P waves and S waves. P waves compress and expand the ground, while S waves move it side to side. Both types of waves travel beneath the surface and cause the ground to move. The student wants to emphasize a similarity between P waves and S waves. Which choice most effectively uses relevant information from the notes to accomplish this goal?