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. a person is standing in the range of the sound waves coming from the original sound sources as well as the reflected waves, c. the reflecting wave source is approximately 21.5 meters from the person, d. a person is standing 30 meters in front of the reflecting surface.

A. A sound wave reaches the ear, and the reflected wave reaches the ear less than 0.1 seconds later,

Explanation:

Human hear can not differentiate two successive sounds when the period between the sounds is less than 0.1 seconds.When the distance between the source of sound and the reflecting surface is less than 17 meters, you can't hear any echo.These two factors go together.If the distance is less than 17 meters, the echo will be less than 0.1 seconds thus the ear won't pick it.