Correct option is C
Vacuum pressure is the pressure that is measured below the atmospheric pressure. It is the difference between the atmospheric pressure and the absolute pressure in a system.
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Absolute pressure: Absolute pressure is the total pressure measured from an absolute zero reference point, not below atmospheric pressure.
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Gauge pressure: Gauge pressure is the pressure above or below atmospheric pressure but is not specifically used to define vacuum pressure.
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Zero pressure line: This refers to absolute zero pressure, which is not relevant to the definition of vacuum pressure.
