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: Turbines are used to producing electricity. 
Question: turbines are used to what?
produce electricity.

Fact: fish need to be cooled. 
Question: what needs to be cooled?
fish.

Fact: the average number of individuals per unit of area or volume can vary considerably. 
Question: the average number of individuals per unit of area or what can vary considerably?

volume.