The Doppler Effect... (Check all that apply.) A) When the fire truck travels towards you the air/air molecules get pushed together by sound waves causing a shorter wavelength and an increased frequency.

B) Occurs whenever there is motion between the source of the sound and the receiver.

C) The pitch of a siren is higher while approaching you and less after it passes.

D) Once the fire truck passes you, the sound waves become more spread out, their wavelength increases, and frequency decreases.