Which of the following describes a condition in which an individual would not hear an echo?

a. an individual is standing 30 meters in front of the reflecting surface.

b. the reflecting source is approximately 21.5 meters from the individual.

c. an individual is standing in the range of the sound waves coming from the original sound source as well as the reflected waves.
d. a sound wave reaches the ear and the reflected wave reaches the ear less than 0.1 second later.