Two Sources, S₁ and S₂, Each Emit Waves of Wavelength $\lambda$

Two sources, S₁ and S₂, each emit waves of wavelength $\lambda$ in the same medium. The phase difference between the two waves, at the point P shown, is . The quantity $\varepsilon$ is:

A) the distance S₁S₂
B) the angle S₁PS₂
C) $\pi$ /2
D) the phase difference between the two sources
E) zero for transverse waves, $\pi$ for longitudinal waves