Why does the frequency of a siren get higher as an ambulance using that siren gets closer? a. as the ambulance gets closer, the sound waves slow down to reach the observer; so the frequency increases. b. as the ambulance gets closer, the decreased distance creates less interference; so the frequency increases. c. as the ambulance gets closer, the frequency of the sound waves is being stretched; so the frequency increases. eliminate d. as the ambulance gets closer, the sound waves are compressed relative to the person; so the frequency increases?