Sources at points P and Q generate two identical waves. These waves follow different routes to arrive at point R, as illustrated. Each wave has a wavelength of $6.0\,\text{cm}$. At point R, the waves are in phase. What phase difference is there between the waves as they leave points P and Q?
- A$0^\circ$
- B$60^\circ$
- C$90^\circ$
- D$120^\circ$