20. Beat is an interference phenomenon when two waves of slightly different frequencies are superposed. How does the beat frequency change when both sound sources (of slightly different frequencies) move relative to the stationary listener? (A) It does not change if both sound sources approach the listener at the same speed. (B) It increases if both sound sources approach the listener at the same speed. (C) It decreases if one sound source approaches and the other moves away from the listener at the same speed. (D) It increases if one sound source approaches and the other moves away from the listener at the same speed. (E) None of the above is false.