Noise reduction headphones create their own sound waves to cancel external sound waves. Inside the headphones, a microphone detects waves of one frequency. A loudspeaker in the headphones then emits a wave with that same frequency but at a different phase. What phase difference is there between the external sound wave and the wave produced by the loudspeaker in the headphones?
- A$90^{\circ}$
- B$180^{\circ}$
- C$270^{\circ}$
- D$360^{\circ}$