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Answer:
Carbondioxide gas.
Explanation:
Carbondioxide gas is actually cycled in a web of organisms, The plants takes carbondioxide gas for the process of photosynthesis, this plant is eaten by herbivores for energy and carbondioxide is released in the process of respiration. After that, carnivores feed on these herbivores and oxygen is used in the breakdown of food for releasing energy and carbondioxide gas is also released from the body through breathing so the cycle of carbondioxide gas is continue.
Answer: B. Water breaks down into oxygen molecules for respiration.
Water is the required for photosynthesis, a process by which plants (terrestrial and aquatic) makes their food by utilizing water as a reactant to produce oxygen and glucose as product. The glucose act as a source of energy for conducting various metabolic processes in plants but oxygen is released in the air.

This oxygen is necessary for terrestrial and aquatic animals for respiration. Therefore, water breaks down into oxygen by the process of photosynthesis, hence water plays important role in respiration in living beings.