Four students make the following statements about sound travelling in air. Identify the incorrect statement.
a) The displacement of air molecules and the pressure variation in a plane sound wave are 90∘ out of phase.
b) Each air molecule travels one full wavelength along the tube in each time period.
c) The distance between a compression and the next rarefaction in air is 2λ.
d) The speed of sound in air increases with rise in temperature.