Consider a line of people, like the line you considered earlier, but closely packed, like particles in a solid. What action could cause an involuntary wave pulse go through this line, and what kind of would it start? The answer below should both describe an involuntary wave and describe the type of wave pulse accurately.

A. The person at the front of the line stretches her body to the side. It would start a transverse wave pulse.
B. Somebody runs into the last person in line, and bumps him accidentally. It would start a transverse wave pulse.
C. The person at the front of the line falls backward into the person behind him. It would start a longitudinal wave pulse.
D. The person at the front of the line stretches her arms out while yawning. It would start a longitudinal wave pulse.