波動學難題!!!!!!!!!!

Translate the longitudinal wave into displacement-distance graph (which is expected to be sinusoidal) as follows:

圖片參考：http://i117.photobucket.com/albums/o61/billy_hywung/April07/Longwave.jpg

(Note: Displacement to the right is taken as positive.)
So comparing the solid waveform (representing the moment shown in the question) and the dotted waveform (representing the moment a little bit later), we can see that, at the respective positions of point Q and Y, they are moving to the left and right respectively at the moment of the solid waveform.
So the answer is D.