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OFFICIAL EXPLANATION
Since the westbound train is blowing its horn, the westbound train is “the source of the sound.” Therefore, the passengers on the westbound train are traveling along with the source of the sound, and thus are neither approaching the source nor receding from it. Hence the westbound passengers should hear the true pitch of the sound.
According to the passage, the passengers in the eastbound train are approaching the source of the sound, the westbound train; the distance between the eastbound passengers and the sound is decreasing. Therefore, according to the given description of the Doppler effect, the eastbound passengers should perceive a sound that is higher than the true pitch.
(A) Passengers in the eastbound train do not hear the true pitch of the horn; passengers in the westbound train do.
(B) It is true that the westbound passengers hear the true pitch; the eastbound passengers, however, hear a sound that is higher than the true pitch.
(C) Passengers in the eastbound train do not hear the true pitch of the horn; passengers in the westbound train do.
(D) CORRECT. Westbound passengers do hear the true pitch of the sound. Eastbound passengers do hear a sound that is higher in pitch than the true sound.
(E) It is true that eastbound passengers hear a sound that is higher than the true pitch; the westbound passengers, however, hear the true pitch.
Actually, the passengers on the west traveling train are on it so they hear the true sound. The passengers on the east train hear a higher sound because the west train is approaching the east.
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