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.

[Q]: Fact: a flashlight requires a source of energy to produce light. 
Question: What requires a source of energy to produce light?
[A]: flashlights.


[Q]: Fact: glass causes bending of light rays. 
Question: what does glass cause?
[A]: bending light rays.


[Q]: Fact: an optical system is used for observing stars by astronomers. 
Question: what is used for observing stars by astronomers?
[A]:
optical system.