Suppose that the separation between two speakers A and B is 5.10 m and the speakers are vibrating in phase. They are playing identical 125-Hz tones, and the speed of sound is 344 m/s. An observer is seated at a position directly facing speaker B in such a way that his line of sight extending to B is perpendicular to the imaginary line between A and B. What is the largest possible distance between speaker B and the observer at C, such that he observes destructive interference?