As the ambulance got closer, Marge noticed that the pitch of the siren got higher. This happened because

A) the sound waves were pushed closer together.

B) the human ear increases pitch over time.

C) the ambulance was further away.

D) the volume increased.

A) the sound waves were pushed closer together

The correct choice is

A) the sound waves were pushed closer together.

Explanation:

As the ambulance gets closer, the distance between the ambulance and marge decreases and so does the wavelength of the siren observed by marge. As the wavelength decrease and speed remains the same for the siren, the frequency of the siren increases because wavelength, frequency and speed are related as

Frequency = Speed/wavelength

Clearly, the frequency is inversely related to wavelength.