Cell, the smallest unit that can exist independently
The correct answer is B. Oxygen.
Oxygen, as a part of the atmosphere, also participates in UV protection, besides being necessary for breathing.
The term metabolism would refer to the chemical reactions that involve the acquisition, storage, or release of energy within cells. It is all the processes involved that would maintain life of organisms. It is divided into catabolism (breaking down of molecules) and anabolism (synthesis of the compounds needed for catabolism).
