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: Viruses are viewed by illuminating electrons. 
Question: What of the following can only be viewed by illuminating electrons?
[A]: viruses.


[Q]: Fact: Some insects communicate with chemicals called pheromones. 
Question: How do some insects communicate?
[A]: pheromones.


[Q]: Fact: Knowledge of the stars can be used as a key element of space travel. 
Question: Knowledge of the stars can be used as what part of space travel?
[A]:
key element.