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.

[EX Q]: Fact: gases from burning coal causes precipitation which pH is 5.7 or lower. 
Question: Gases from what causes precipitation where the pH is 5.7 or lower?
[EX A]: burning coal.

[EX Q]: Fact: memory cells help keep people from getting sick. 
Question: What helps keep people from getting sick?
[EX A]: Memory cells.

[EX Q]: Fact: Light causes images to be retained on the retina. 
Question: Where are images retained?
[EX A]:
Retina.