While you are standing on the sidewalk, an ambulance traveling at 18.00 m/s passes you. The ambulance’s siren produces a tone at a frequency of 848.0 Hz. The speed of the sound in the air is 343.0 m/s. What frequency do you hear as the ambulance approaches, assuming the speed of sound remains constant?

a) 840.8 Hz
b) 848.0 Hz
c) 855.2 Hz
d) 862.4 Hz