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

Q: Passage: Abundance The elements vary widely in abundance. In the universe as a whole, the most common element is hydrogen (about 90% of atoms), followed by helium (most of the remaining 10%). All other elements are present in relatively minuscule amounts, as far as we can detect. On the planet Earth, however, the situation is rather different. Oxygen makes up 46.1% of the mass of Earth’s crust (the relatively thin layer of rock forming Earth’s surface), mostly in combination with other elements, while silicon makes up 28.5%. Hydrogen, the most abundant element in the universe, makes up only 0.14% of Earth’s crust. Table 2.1 "Elemental Composition of Earth" lists the relative abundances of elements on Earth as a whole and in Earth’s crust. Table 2.2 "Elemental Composition of a Human Body" lists the relative abundances of elements in the human body. If you compare Table 2.1 "Elemental Composition of Earth" and Table 2.2 "Elemental Composition of a Human Body", you will find disparities between the percentage of each element in the human body and on Earth. Oxygen has the highest percentage in both Saylor URL: http://www. saylor. org/books. Answer: hydrogen

A: In the universe as a whole, what is the most common element?
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Q: Passage: Refracting and reflecting telescopes are optical telescopes that use lenses to gather light. Answer: light

A: Refracting and reflecting telescopes are optical telescopes that use lenses to gather what?
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Q: Passage: Biofuels are useful because they are liquid. Biofuels can go into a gas tank unlike many other types of alternative energy. Answer: liquid

A:
What form do biofuels take that makes them very useful?
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