instruction:
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.
question:
Fact: space exploration is not possible without navigation. 
Question: What is used for space exploration?
answer:
navigation.


question:
Fact: Evolution occurs by a sorting mechanism process. 
Question: How does evolution occur?
answer:
A sorting mechanism process.


question:
Fact: Viruses do not have walls and membranes. 
Question: What lacks walls and membranes?
answer:

viruses.