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. an individual is standing in the range of the sound waves coming from the original sound source as well as the reflected waves.
c. a sound wave reaches the ear and the reflected wave reaches the ear less than 0.1 second later.
d. the reflecting source is approximately 21.5 meters from the individual.

If the echo (the reflected sound) reaches your ear less than about
0.1 second after the original sound, your brain doesn't separate them,
and you're not aware of the echo even though it's there.

If the echo comes from, say, a wall, 0.1 second means you'd have to be
about  17 meters away from the wall.  If you're closer than that, then the
echo reaches you in less than 0.1 second and you're not aware of it.

A. 30 meters . . .
     No.  You hear that echo easily

B.  you're standing within range of both sounds . . .
     No. You hear that echo easily, if you're at least 17 meters from the wall.

C.  less than 0.1 second later . . .
     That's it.  The echo is there but your brain doesn't know it.

D.  21.5 meters
     No.  You hear that echo easily.

The correct answer is c