A police car, with its siren on, is motionless in heavy traffic. A pedestrian notices that, even though the loudness of the sound stays the same, the pitch changes. Which row states the amplitude and frequency of the sound?
- Aamplitude: constant; frequency: constant
- Bamplitude: constant; frequency: varying
- Camplitude: varying; frequency: constant
- Damplitude: varying; frequency: varying