Question

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blood brings carbon dioxide to the lungs, where it also picks up needed by cells of the body.
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Explanation:

The process of gas exchange in the lungs is a fundamental biological concept. Blood, which has carried carbon dioxide (a waste product) to the lungs, undergoes a gas - exchange process. In the alveoli of the lungs, carbon dioxide diffuses out of the blood into the alveolar space, and oxygen ($O_2$) diffuses from the alveolar space into the blood. Oxygen is then transported by the blood (bound to hemoglobin in red blood cells) to the body's cells, which need it for cellular respiration (the process of generating energy in the form of ATP).

Answer:

Oxygen