A loudspeaker producing a sound wave of one frequency is positioned a distance $L$ from a reflecting surface. A stationary wave forms with an antinode at the loudspeaker. A microphone is then moved from the loudspeaker towards the reflector. Before the microphone reaches the reflector, it detects four positions where the sound intensity is at a minimum. What is the wavelength of the sound wave?
- A$\frac{2L}{9}$
- B$\frac{2L}{8}$
- C$\frac{4L}{9}$
- D$\frac{4L}{8}$