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.