You are in a vehicle heading due west at a speed v. an ambulance is travelling due east toward you at the same speed with the siren on at 900hz. Which of the following is true?
1) The frequency you hear will remain at 900hz.
2) The frequency you hear will be higher than 900hz.
3) The frequency you hear will be lower than 900hz.
4) The frequency you hear will depend on the exact relative position of the ambulance.