What is the best description of sound waves?

a. a source vibrates up and down, causing air molecules to move up and down perpendicular to the direction the wave is transmitted
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b. a source vibrates back and forth, causing air molecules to move back and forth in the same direction that the wave is transmitted.

c. a source vibrates up and down and back and forth, causing air molecules to move in a circular motion as the wave is transmitted.

d. a source ejects a ring of high-velocity air molecules that travel from the source to a listener.