The diagram below shows various positions of a child in motion on a swing. Somewhere in front of the child a stationary whistle is blowing. At which position will the child hear the highest frequency for the sound of the whistle?
[Figure: Swing diagram showing positions A, B, C, D with whistle in front]
(a) At B when moving toward A.
(b) At B when moving toward C.
(c) At C when moving toward B.
(d) At C when moving toward D.