11. How do longitudinal waves occur?

A. It occurs when the individual particles of a medium vibrate up and down in the direction in which the waves travel.

B. It occurs when the individual particles of a medium vibrate side by side to the direction in which the waves travel.

C. It occurs when the individual particles of a medium vibrate back and forth in the direction in which the waves travel.

D. It occurs when the individual particles of a medium vibrate above to the direction in which the waves travel. ​