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(A) Sound waves from the siren remain constant. As the police car drives past Sam, the sound waves decrease in frequency.
(B) The frequency of sound waves from the siren increase. As the police car continues past Sam, the sound waves decrease in frequency.
(C) The frequency of sound waves from the siren decrease. As the police car drives past Sam, the sound waves increase in frequency.
(D) The frequency of the sounds waves from the siren increase and continues to increase as the police car passes and drives away from Sam.
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i think that is the best option, but....
looking at the possible answer i don't think they are well expressed, since the freq of the siren is always the same!!! the person that hears the sound perceives a modified frequency due to the movement of the source.
but again if i had to pick one of those i would go with B