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: An automobile engine works by summing kinetic energy and potential energy. 
Question: What works by summing kinetic energy and potential energy?
an automobile engine.

Fact: The sun causes clouds to form. 
Question: What causes water to evaporate and clouds to form?

The sun.