The diagram illustrates equipment used to determine the frequency of a sound wave. A sound wave with an unknown frequency is bounced back from a metal plate. A microphone positioned a distance $D$ from the metal plate measures the sound intensity. A minimum intensity is observed when $D = 12.0\ \text{cm}$. The plate is then shifted farther from the microphone until the next minimum is observed when $D = 15.0\ \text{cm}$. The speed of sound in air is $336\ \text{m s}^{-1}$. What is the frequency of the sound?
- A$56\ \text{Hz}$
- B$112\ \text{Hz}$
- C$5600\ \text{Hz}$
- D$11\,200\ \text{Hz}$