The top surface of a calm, smoothly flowing stream is always at atmospheric pressure. As water in this stream runs into a tree stump and slows almost to a stop, the water's top surface
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shifts upward slightly above the normal stream level.
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shifts downward slightly below the normal stream level.
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stays at the same height and does not begin to rotate.
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stays at the same height but begins to rotate clockwise as viewed from above.
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