Given a scientific passage and an answer, generate a question for the given answer.

Q: Passage: Carbon has four valence electrons, so it can achieve a full outer energy level by forming four covalent bonds. When it bonds only with hydrogen, it forms compounds called hydrocarbons. Answer: four

A: How many valence electrons does carbon have?
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Q: Passage: A cross section of earthquake epicenters. The depth outlines the subducting plate. There are shallow, intermediate, and deep earthquakes. Answer: earthquake

A: Subducting plate and epicenter are terms related to what phenomenon?
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Q: Passage: Natural resource that is limited in supply and cannot be replaced except over millions of years. Answer: natural resource

A:
What type of resource is limited in supply and cannot be replaced except over millions of years?
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