Definition: a gas that is in the air and that animals need to live Level: B2
消防员在烟雾弥漫的房间里使用了氧气面罩。
The firefighter used an ___oxygen mask in the smoky room.
植物在光合作用中释放氧气作为副产品。
Plants release ___oxygen as a byproduct of photosynthesis.
氧气约占地球大气的21%。
___Oxygen makes up about 21% of the Earth's atmosphere.
