Two identical whistles emit the same rest frequency while moving on a straight track. Whistle A travels at 8m⋅s−1 toward an observer, and Whistle B travels at 12m⋅s−1 away from the same observer. Ignoring wind, which of the following best describes the frequencies heard by the observer?
(a) Both whistles are heard at their rest frequency because they move in opposite directions.
(b) Both whistles produce the same frequency shift since only their direction of motion differs.
(c) The frequency from Whistle A is higher than the rest frequency, while the frequency from Whistle B is lower.
(d) The frequency from Whistle A is lower than the rest frequency, and the frequency from Whistle B is higher.