B is the room where you will get t he effect of echo.
Explanation:
The minimum distance required to get the echo effect is 17.2 m.
The echo effect is produced when there is a reflectionof sound waves.
When you say something, the sound waves leave your mouth. It travels a minimum distance, 17.2 m to produce an echo. It hits a hard surface and hence, it bounces back. These sound waves will come back to your ear. So, you could hear what you said a second ago.
The velocity of sound waves in dry air is 343 m at 25 degree Celsius temperature.
