A glass tube placed horizontally is sealed at one end, and a layer of dust has been spread along the inside bottom surface. A loudspeaker is positioned close to the open end of the tube and sends out sound. The sound frequency is adjusted, and at one particular frequency a stationary wave is produced in the tube, causing the dust to collect into small heaps. The separation between four heaps of dust is $30\,\text{cm}$. The speed of sound in the air inside the tube is $330\,\text{m s}^{-1}$. What frequency does the loudspeaker emit?
- A$1650\,\text{Hz}$
- B$2200\,\text{Hz}$
- C$3300\,\text{Hz}$
- D$6600\,\text{Hz}$