Which statement does not explain the concept of the doppler effect? a) as an observer approaches a stationary sound, the sound waves are compressed, making the pitch higher. b) as an observer moves away from a stationary sound, the sound waves are stretched making the pitch lower. c) as a sound source approaches a stationary object, the sound waves are compressed, making the pitch higher. d) as a sound source moves away from a stationary object, the sound waves are compressed, increasing the pitch.