(i) Define an echo.
(ii) Explain, with reference to the persistence of hearing, why a gap of at least 0.1 s between the direct sound and its reflection is necessary for an echo to be perceived as a separate sound.
(iii) Using the information in part (ii), show that at 25 °C the reflecting surface must be at least about 17 m from the source for an echo to be heard distinctly. Assume the speed of sound in air at 25 °C is 346 m s⁻¹.