Two separated sources emit sinusoidal traveling waves that have the same wavelength λ and are in phase at their respective sources. One travels a distance f1 to get to the observation point while the other travels a distance f2. The amplitude is a minimum at the observation point if f1 − f2 is: A. an odd multiple of λ/2 B. an odd multiple of λ/4 C. a multiple of λ D. an odd multiple of π/2 E. a multiple of π