We hear an echo due to

Which of the following animals can hear the highest frequencies?

How much should minimum distance be between the source of sound and reflecting surface, so that an echo can be heard clearly?

An echo is heard after 5 seconds of the production of sound which moves with a speed of 340 m/s. What is the distance of the mountain from the source of sound which produced the echo?

What is the audible range (hearing range) of humans?

A worker was working on the rail track. A boy at a distance holds his ear near the rail track. This boy was able to hear sound of the blow of workman twice. It is because of :