Which of the following is most likely to happen when a sound wave reflects off a hard surface? (Assume that neither the source of the sound nor the hard
surface is moving.)
A. A change in speed
B. An echo
C. A change in pitch
D. A beat frequency

В

an echo is caused by the fact that waves can be reflected by solid surfaces, this is due to the dynamic pattern of rarefactions and air seals near the reflecting surface

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that's what i got you can count the number of atoms