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

Q: Passage: Natural gas is delivered to homes, where it is used for cooking and heating. Natural gas is also a major energy source for powering turbines to make electricity. Natural gas releases most of its energy as heat when it burns. The power plant is able to use this heat, either in the form of hot gases or steam, to spin turbines. The spinning turbines turn generators, and the generators create electricity. Answer: natural gas

A: What type of gas is delivered to homes to use for cooking and heating?
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Q: Passage: An ionic bond is the force of attraction that holds together oppositely charged ions. Ionic bonds form crystals instead of molecules. Table salt contains ionic bonds. Answer: ionic bond

A: What term means the force of attraction that holds together oppositely charged ions?
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Q: Passage: Magma that cools underground forms intrusions ( Figure below ). Usually this magma is very viscous felsic magma. This magma does not rise easily and so cools slowly underground. Intrusions become land formations if they are exposed at the surface by uplift and erosion. Answer: intrusions

A:
What does magma that cools underground form?
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