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.

Aparticle's position is given by x = 3.00 - 9.00t + 3t2, in which x is in meters and t is in seconds. (a) what is its velocity at t = 1 s? (b) is it moving in the positive or negative direction of x just then? (c) what is its speed just then? (d) is the speed increasing or decreasing just then? (try answering the next two questions without further calculation.) (e) is there ever an instant when the velocity is zero? if so, give the time t; if not, answer "0". (f) is there a time after t = 3 s when the particle is moving in the negative direction of x? if so, give the time t; if not, answer "0".