In which of the following conditions would a person not hear an echo?
a. a sound wave reaches the ear, and the reflected wave reaches the ear less than 0.1 seconds later.
b. the reflecting source is approximately 21.5 meters from the person.
c. a person is standing 30 meters in front of the reflecting surface.
d. a person is standing in the range of the sound waves coming from the original sound source as well as the reflected waves.