Which one of the following best explains why an echo is usually not heard in an ordinary furnished classroom?
(a) Most of the sound energy is absorbed by curtains, desks and people, so the reflected sound is too weak to be heard.
(b) The reflected sound reaches the ear after about 0.5s, which is considered too late to be recognised as an echo.
(c) The reflected sound returns in less than 0.1s, so it merges with the direct sound.
(d) The reflected sound waves undergo destructive interference with the incident waves, cancelling the echo completely.