Beats are produced because the two waves:

  1. are polarized
  2. travel in opposite directions
  3. have different speeds
  4. drift in and out of phase
Answer

D) drift in and out of phase

Alternate constructive and destructive interference. SLO P-11-D-15 · Understanding

Physics MCQ · More Superposition, Stationary Waves and Beats MCQs · Practise