b. Doppler principle.
Doppler ultrasound measures the velocity of any reflectant object/fluid by evaluating the change of frequency of the reflected ultrasound. You have heard a Doppler frequency shift. When a whistling train approaches, you hear the "eeee" sound. As soon as it passes, the frequency of the whistle suddenly drops to a "oooo." This change to a lower pitch is because approaching objects compress the sound waves to make them sound higher in pitch. Objects moving away, rarify reflected sound waves and thus sound lower in pitch.