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.

[EX Q]: Fact: cell regeneration requires rest. 
Question: what requires rest?
[EX A]: cell regeneration.

[EX Q]: Fact: Food is used for energy by the cells of most organisms. 
Question: Food is used for energy by what part of most organisms?
[EX A]: Cells.

[EX Q]: Fact: Algae play significant roles as producers in water-based ecosystems. 
Question: _ play significant roles as producers in water-based ecosystems.?
[EX A]:
algae.