The correct answer is a and b both are correct. Explanation & Key Takeaways: • The whistle of a passing train is clearly heard on a quiet misty night. Since the speed of sound depends on the density of the medium. • The velocity of sound in air is proportional to humidity in the air. • An increase in humidity causes an increase in the density of the air. • Because of humidity, there is a 0.1 to 0.6 times change in the speed of sound. • The speed of sound in an ideal gas doesn't depend on the pressure and density of the medium. • The individual contribution of these two factors in determining the speed of sound is countered by each other. • Hence both the statement are correct. Related Concepts: • A sound is a vibration that propagates through a medium in the form of a mechanical wave. • The medium in which it propagates can either be a solid, a liquid or a gas. • Sound travels fastest in solids, relatively slower in liquids and slowest in gases. • A sound wave is the pattern of disturbance caused by the energy travelling away from the source of the sound. • Sound waves are longitudinal waves. • This means that the propagation of vibration of particles is parallel to the direction of the energy wave propagation. • When the atoms are set in vibration they move back and forth. • This continuous back and forth motions result in a high-pressure and a low-pressure region in the medium. • These high-pressure and low-pressure regions are termed as compressions and rarefactions, respectively.