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: A bill is used for catching prey by some birds. 
Question: What is used by some birds to catch their prey?

A: a bill.
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Q: Fact: large animals that roamed the earth were replaced with mammals after their extinction. 
Question: large animals that roamed the earth were replaced with what after their extinction?

A: mammals.
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Q: Fact: Passive immunity lasts only as long as the proteins that attach to an antigen survive in body fluids. 
Question: What lasts only as long as the proteins that attach to an antigen survive in body fluids?

A:
passive immunity.
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