Which of the following is an example of the Doppler effect? (2 points)
A. A water bug on the surface of a pond is producing small ripples in the water. The bug is not moving
B. A police car with its siren on is driving towards you, and you perceive the pitch of the siren to increase.
C. Scientists measure the speed of light and find that it is constant.
D. The gravitational pull of two objects on each other
