Two Loudspeakers Are Placed Facing Each Other and Driven by the Same

Multiple Choice

Two loudspeakers are placed facing each other and driven by the same source at 680 Hz.A listener is positioned between the two speakers on the line separating them,thus creating a constructive interference at the listener's ear.If one of the speakers is gradually pushed toward the listener,how far must it be moved to repeat the condition of constructive interference at the listener's ear? (The speed of sound = 340 m/s. )

A) 0.13 m
B) 0.25 m
C) 0.50 m
D) 1.00 m