Concept: In a non-reacting gas mixture, each gas exerts its own independent pressure.Explanation: When gases are mixed without any chemical reaction, they do not affect each other's pressure.The pressure of a single gas in that mixture is called its partial pressure.Dalton's law of partial pressures states that the total pressure equals the sum of the partial pressures of all gases.Therefore, the pressure of an individual gas is not atmospheric pressure, vapor pressure, or the total pressure.Answer: D. partial pressure