Which of the Following Observations Is a Result of the Doppler

Which of the following observations is a result of the Doppler effect?

A) Noise from across a lake is louder at night than during the day
B) A person walking notices that the pitch of a car's engine decreases as the car passes by.
C) Beats are produced when two tuning forks with different frequencies are heard together.
D) A person hears the sound from a radio more clearly in certain areas of a room than in others.