The echo (reflected sounD. will be distinctly heard only at ordinary temperatures if the distance of the reflecting surfaces from the source of sound is at least ________?

The echo (reflected sounD. will be distinctly heard only at ordinary temperatures if the distance of the reflecting surfaces from the source of sound is at least ________?

A. 1120 ft
B. 120 ft
C. 56 ft
D. 100 ft
E. None of these