The speed of a wave depends on the
a. crest
c. medium
b. pitch
d. amplitude

Wave is a disturbance in a medium that transmits the energy between the two points. The medium is the material that wave travels through like water, air or gas. As the wave propagate, they disturb the density of the medium.

The medium and the speed of wave has a direct relationship. The higher the density of the medium, the higher the speed and as the density of medium goes lower so as the speed of the wave. The speed of the wave depends on the density of the medium.
C. Medium

I'm pretty sure the answer is c 

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