Which of the following situations results in the observer detecting the same frequency as that emitted by the sound source?
(a) Both the source and the observer move eastward at 15m/s along the same straight line.
(b) The source moves eastward at 15m/s and the observer moves westward at 15m/s along the same line.
(c) Both the source and the observer are at rest, but a strong wind blows westward at 15m/s between them.
(d) The source is at rest while the observer moves westward toward it at 15m/s.