As a fire truck approached mary, the pitch of the siren got higher. once the fire truck passed and moved away, the sirens pitch got lower. this change in pitch is because in the sound waves

The answer is; frequency

The change in pitch is due to changes in frequency. As the fire engine gets closer, the pitch is higher because of the increased frequency (akin to compression of waves). As the fire engine gets further, the pitch decreases - akin to elongation of the waves. This phenomenon is akin to the Doppler effect of light in space.