When beats are produced by two progressive waves of nearly the same frequency, which one of the following is correct?

• A. The particles vibrate simple harmonically, with the frequency equal to the difference in the component frequencies.
• B. The amplitude of vibration at any point changes simple harmonically with a frequency equal to the difference in the frequencies of the two waves.
• C. The frequency of beats depends upon the position, where the observer is.
• D. The frequency of beats changes as the time progresses.