The figure below shows four isotropic point sources of sound that are uniformly spaced on an x axis. The sources emit sound at the same wavelength λ and same amplitudes, and they emit in phase. A point P is shown on the x axis. Suppose that as the sound waves travel to P, the decrease in their amplitude is negligible. What multiple of sm is the amplitude of the net wave at P if distance d in the figure is (a) 1λ,(b) 2λ, and (c)4λ?