Doppler's effect:
The Doppler effect states that the frequency of sound increases as it source approaches you and it decreases as the source turns away from you.
When the ambulance drives past you, the pitch of the siren is lower. This is due to the fact that the frequency of the wavelength decreases.
A higher pitch means frequency increases and a lower pitch means frequency decreases.
Therefore, the correct answer is option B. decreases
