Two particles are executing simple harmonic motion of the same amplitude $A$ and frequency $\omega$ along the $x$ -axis. Their mean position is separated by distance $X_0$ ($X_0>A$). If the maximum separation between them is $(X_0+A)$, the phase difference between their motion is:

1. A
   $\frac{\pi }{3}$
2. B
   $\frac{\pi }{4}$
3. C
   $\frac{\pi }{6}$
4. D
   $\frac{\pi }{2}$