Which of the following best explains why sound travels faster in warm air than in cold air?
(a) Warm air is less dense, so there are fewer molecules to impede the motion of the sound source.
(b) The average kinetic energy of air molecules is higher, shortening the time between successive molecular collisions that transmit the disturbance.
(c) Thermal expansion of air increases the wavelength while the frequency stays fixed, automatically raising the wave speed.
(d) Warm air contains more water vapor; because water molecules are lighter, the mixture’s average molecular mass is lower, and the speed v=γRT/M therefore increases.