Jenny and Fred both play a long note on their trombones. A stationary observer hears the sound increasing and decreasing repeatedly in loudness. Which of these must be true?
A. The sound waves from the two trombones have slightly different frequencies.
B. The sound waves from the two trombones have the same frequency.
C. The sound waves from the two trombones have slightly different amplitudes.
D. The sound waves from the two trombones have the same wavelength.
E. The sound waves from the two trombones have slightly different velocities.