At one instant a sound wave traveling through air is sampled along a line. The measured pressure varies with distance and shows labeled points X at the highest pressures and Y at the lowest pressures. Which statement correctly identifies X and Y?
a) Points X are compressions, and points Y are rarefactions.
b) Points X are rarefactions, and points Y are compressions.
c) Points X and Y are nodes where air particles do not move, because pressure maxima correspond to zero displacement.
d) Points X correspond to crests and points Y to troughs, because sound waves are transverse in nature.