Use the drop-down menus to select the best answers.Carbon dioxidePlasmaOxygenBicarbonate ion is a life-sustaining, odorless gas in the air.

Oxygen is the life-sustaining, odorless gas in the air that humans and many other organisms use for respiration. Carbon dioxide is also a gas in the air, but it is not the gas described as necessary for breathing; plasma and bicarbonate ions are not gases.

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