A student claps once near a high wall but does not hear any echo when standing only 5m from it. Which statement best explains this observation?
a) All the sound energy is completely absorbed by the wall at such a short distance.
b) Destructive interference cancels the reflected sound when the distance is small.
c) The time gap between the sound and its reflection is less than 0.1s, so the ear cannot distinguish them.
d) The reflected sound is too weak because intensity falls rapidly with distance, so it becomes inaudible.