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.


[Q]: Fact: Blowing air into a whistle causes air to move inside the whistle. 
Question: Blowing air into what causes air to move inside it?
[A]: whistle.


[Q]: Fact: Scientists still know relatively little about prokaryotes. 
Question: Scientists still know relatively little about what?
[A]:
prokaryotes.