Two Separated Sources Emit Sinusoidal Traveling Waves That Have the Same

Two separated sources emit sinusoidal traveling waves that have the same wavelength $\lambda$ and are in phase at their respective sources.One travels a distance $\lambda$ ₁ to get to the observation point while the other travels a distance $\lambda$ ₂ The amplitude is a maximum at the observation point if $\lambda$ ₁ $\lambda$ ₂ is:

A) an odd multiple of $\lambda$ /2
B) an odd multiple of $\lambda$ /4
C) a multiple of $\lambda$
D) an odd multiple of $\pi$ /2
E) a multiple of $\pi$