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: the key cells involved in the immune response are produced in the bone marrow. 
Question: where are the key cells involved in the immune response produced in?
answer:
bone marrow.


question:
Fact: Blowing air into a whistle causes air to move inside the whistle. 
Question: Blowing air into what causes air to move inside it?
answer:
whistle.


question:
Fact: humans require stamina to succeed at physical activities. 
Question: what requires stamina to succeed at physical activities?
answer:

humans.