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: vein deposits of gold and silver are formed by granite. 
Question: what are vein deposits of gold and silver formed by?
granite.

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

Fact: seconds are used to measure a period. 
Question: What is used to measure a period?

seconds.