Two coherent sources of sound, S1 and S2, produce sound waves of the same wavelength, λ=1 m, in phase. S1 and S2 are placed 1.5 m apart (see fig.). A listener, located at L, directly in front of S2, finds that the intensity is at a minimum when he is 2 m away from S2. The listener moves away from S1, keeping his distance from S2 fixed. The adjacent maximum of intensity is observed when the listener is at a distance d from S1. Then, d is:
(a) 12m
(b) 5m
(c) 2m
(d) 3m