The Doppler Effect. When a train is approaching, the sound waves in front of it are "squished" and the sound of a train whistle will seem to be a higher pitch than the actual pitch of the whistle. If the train is departing, the sound waves will appear to be "stretched" and the sound will be lower than the actual pitch of the whistle. As the train passes by, the sound will shift from the higher pitch to the lower pitch, passing its true sound on the way.
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The correct answer is agriculture. It was made possible for people to live on a single piece of land and not be nomadic anymore in search of food. Agriculture enabled them to settle around river valleys like Nile or Tigris and Euphrates. This is the era when civilizations started rising worldwide.
We, the United States, declare ourselves free from Great Britain. We grant ourselves all the powers that a free nation should have. We shall work together to achieve this goal.
