In which of the following conditions would a person not hear an echo? A. a sound wave reaches the air and the reflected wave reaches they are less than 0.1 seconds later.
B. a person is standing 30 m in front of the reflecting surface.
C. the reflecting source is approximately 21.5 meters from the person.
D. a person is standing in the range of the sound waves coming from the original sound source as well as the reflected waves.