Given a scientific passage and an answer, generate a question for the given answer.
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Question: Passage: Biofuels are useful because they are liquid. Biofuels can go into a gas tank unlike many other types of alternative energy. Answer: liquid

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

Question: Passage: Secondary pollutants form from chemical reactions that occur when pollution is exposed to sunlight. Answer: sunlight

Answer: Secondary pollutants form from chemical reactions that occur when pollution is exposed to what?

Question: Passage: Glaciers cause erosion by plucking and abrasion. Valley glaciers form several unique features through erosion, including cirques, aretes, and horns. Answer: valley glaciers

Answer:
Which type of glaciers form several unique features through erosion, including cirques, aretes, and horns?