instruction:
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.
question:
Fact: sharks have tails. 
Question: What has a tail?
answer:
sharks.


question:
Fact: conserving water can be used for survival in a desert. 
Question: Where is conserving water useful for survival?
answer:
desert.


question:
Fact: passive immunity lasts only as long as the antibodies survive in blood. 
Question: passive immunity lasts only as long as the antibodies survive where?
answer:

blood.