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: Some insects communicate with chemicals called pheromones. 
Question: How do some insects communicate?
pheromones.

Fact: if a gas is heated the density decreases. 
Question: What happens to the density of a gas when heated?
decreases.

Fact: burning coal increases the carbon in the atmosphere. 
Question: What does burning coal increase carbon in?

the atmosphere.