Sam is walking through the park. He hears a police car siren moving on the street coming toward him. What happens to the siren sound as it approaches and then passes Sam?
a. As the siren approaches Sam, the frequency of sound waves from the siren increase. As the police car continues past Sam, the sound waves decrease in frequency.
b. As the police car approaches Sam, sound waves from the siren remain constant. As the car continues past Sam, the sound waves decrease in frequency.
c. As the police car approaches Sam, the frequency of sound waves from the siren decrease. As the car continues past Sam, the sound waves increase in frequency.
d. As the police car approaches Sam, the frequency of the sounds waves from the siren increases and continues to increase as the car passes and moves away from Sam.