Question

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

Answer 1

D. A sound wave reaches the ear and the reflected wave reaches the ear less than 0.1 second later.

Just took the Penn Foster test, got it right. =]

Answer 2

The right answer is D. In a situation where the sound wave reaches the ear and the reflected wave reaches the ear less than 0.1 seconds later, the individual would not be able to hear an echo. There needs to a far more significant delay between the sound and the reflection of said sound reaching the listener's ear for the echo effect to become apparent.