In air, the minimum distance between the source and the reflector for an echo to be heard is around (b) 17 m.
The minimum distance between the source and the reflector in air, so that an echo is heard is approximately:
(a) 10 m
(b) 17 m
(c) 34 m
(d) 50 m
(a) 10 m
(b) 17 m
(c) 34 m
(d) 50 m