17. What kind of wave require a medium to travel through? A. mechanical wave B. electromagnetic wave C. microwave D. radio wave 18. is a substance or material through which a wave can propagate or produce A structure B. medium C. object D. body 19. What is the lowest point of a wave? A. amplitude B. trough C. crest D. wavelength 20. It is the height of a wave and also measures its intensity. A. crest B. frequency C. amplitude D. trough