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.

Fact: the average number of individuals per unit of area affect the balance of the environment. 
Question: The average number of individuals per unit of area affect the balance of .?
the environment.

Fact: A fiber found only in mammals grows from a bulb under the skin. 
Question: A fiber found only in mammals grows from a bulb under what?
the skin.

Fact: foam materials can be used to keep warm. 
Question: foam materials can be used to keep what?

warm.