oxygen

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A colorless, odorless gas essential for life, making up about 21% of Earth's atmosphere.

• Humans need oxygen to breathe.
• Plants produce oxygen through photosynthesis.
• The patient was given oxygen to help him breathe.
• Marine biologists discovered that declining oxygen levels in coastal waters have created dead zones where most aquatic species cannot survive.
• The astronauts relied on a sophisticated life-support system to recycle oxygen and remove carbon dioxide during their six-month mission aboard the space station.
• Researchers found that increased oxygen concentrations in the Cambrian period may have facilitated the rapid diversification of complex multicellular organisms.