Liying Nursery is known as "the natural oxygen bar in the garden behind the city". This is mainly because plants carry out () A. respiration B. transpiration C. photosynthesis.

Plants breathe in oxygen and exhale carbon dioxide. A mistake.

B, transpiration is the water that plants lose to the atmosphere, not oxygen, b wrong.

Analysis shows that green plants can not only produce organic matter, but also provide oxygen for other organisms. C is correct.

D there are conduits and sieve tubes in plants, which can transport organic matter, water and inorganic salts, and have nothing to do with oxygen. D error.

So choose: C.