A boy claps his hands while standing 12\,m in front of a high wall. The speed of sound in air is 340\,m/s. What will he experience?
(a) He will hear an echo after about 0.07s.
(b) He will hear an echo after about 0.14s.
(c) He will not hear a distinct echo because the time gap is too small.
(d) He will hear an echo after about 0.035s.