Two Loudspeakers 6

Two loudspeakers 6.00 m apart emit the same frequency tone in phase at the speakers. A listener notices that the intensity of the sound is minimized when she is placed 8.00 m in front of the second speaker. The lowest frequency of the emitted tone (assume the speed of sound in air is 343 m/s) is

A) 1.03 kHz.
B) 686 Hz.
C) 343 Hz.
D) 172 Hz.
E) 85.8 Hz.