How does the "two sources" experiment prove that sound is indeed a wave?
A. I could hear the sound. Like the old adage: "Hearing is believing."
B. The wave moves vertically. This was verified by moving the listener through the crests and troughs of the vertical wave.
C. At the points where we hear no sound, the crest of one wave is canceling the trough of the other wave.
D. Having crests and troughs is a characteristic of waves.