(N/A) The intensity of sound increases due to two main reasons:
$(i)$ The amount of water vapor present in the atmosphere is significantly higher on a rainy day than on a dry day. Since water vapor is lighter than dry air,the density of moist air becomes less than that of dry air. As the speed of sound is inversely proportional to the square root of the density,the velocity of sound increases due to the presence of moisture.
$(ii)$ Because of the higher velocity in moist air,the sound travels the distance in a comparatively shorter time. Consequently,the air resistance acts for a shorter duration,leading to a smaller decrease in amplitude,which results in higher intensity.
$(b)$ Rajan's observation reflects a deep knowledge of the subject and the ability to apply scientific fundamentals to real-life situations.