Write a correct answer to the given question based on its associated fact. Make sure that your answer is contained in the associated fact. Don't be creative and introduce any new word that is not mentioned in the associated fact! Remember that the associated fact has been rearranged to form the question. So, the correct answer words must lie within the associated fact. The correct answer can be a word, phrase, or even a sentence.

Q: Fact: The mass of the Earth helps cause flowing rivers. 
Question: Which aspect of the Earth is needed to have flowing rivers?

A: Mass.
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Q: Fact: Vertebrates have a complete digestive system and a closed circulatory system. 
Question: vertebrates have a complete digestive system and a what?

A: closed circulatory system.
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Q: Fact: electron transport is the final stage of a process that uses oxygen and releases carbon. 
Question: what is process is electron transport the final stage of?

A:
use oxygen, release carbon.
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