Phy★longitudinal wave★particle

Try to look at the animation on the following web-page. You could freeze the motion by pressing the [stop] button, and view it picture by picture using the buttons [>>] and [<<] on the two sides.


http://www.cbu.edu/~jvarrian/applets/waves1/lontra_g.htm

The answer is certainly not option C. It should be option A. Particle A is moving towards the left. Particle A is the point where rarefaction occurs.
If you transform the displacement of each particle into a dispacement-distance curve, a sine-curve is obtained. You will find that (i) those particles having positions on the sine-curve with negative slope are moving towards the right (assume the wave is travelling from left to right), whereas particles having positions on the sine-curve with positive slope are moving towards the left. (ii) Particles at the crest or trough of the sine-curve (i.e. zero slope, hence zero velcoity) are momentarily at rest. (iii) Since the slope on a sine-curve changes sign (from +ve to -ve or vice versa) at the crest or trough, this indicates that particles on the two sides of a particle that is momentarily at rest are moving in opposite directions. (iv) It then follows that particles at the crest are changing direction from right to left, whereas particles on the trough are changing direction fromleft to right .

With the above understanding, try to view the animation picture by picture carefully. You should be able to understand the phenomenon clearly.