A red car and a blue car can move along the same straight one-lane road. Both cars can move only at one speed when they move (e.g., 60 mph). The driver of the red car sounds his horn. In which one of the following situations does the driver of the blue car hear the highest horn frequency?
(a) Both cars are moving at the same speed, and they are moving apart.
(b) Both cars are moving in the same direction at the same speed.
(c) Both cars are moving at the same speed, and they are moving toward each other.
(d) The red car is moving toward the blue car, which is stationary.
(e) The blue car is moving toward the red car, which is stationary.