A fire engine moving to the right at 40 m/s sound its horn (frequency = 500 Hz) at the two vehicles shown in the figure. The car is moving to the right at 30 m/s, while the van is at rest. (a) What frequency is heard by the passenger in the car? (b) What is the frequency as heard by the passengers in the van? (c) When the fire engine is 200 m from the car and 250 m from the van, the passengers in the car hear the sound intensity level of 90 dB. At that moment, what intensity level is heard by the passenger in the van?