Given a scientific passage and an answer, generate a question for the given answer.
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Question: Passage: A snowflake is made of ice, or water in the solid state. A solid is one of four well-known states of matter. The other three states are liquid, gas, and plasma. Compared with these other states of matter, solids have particles that are much more tightly packed together. The particles are held rigidly in place by all the other particles around them so they can't slip past one another or move apart. This gives solids a fixed shape and a fixed volume. Answer: plasma

Answer: What state of matter completes the list: solid, liquid, gas?

Question: Passage: In this equation, the letters A and B represent the reactants that begin the reaction, and the letter C represents the product that is synthesized in the reaction. The arrow shows the direction in which the reaction occurs. Answer: reaction mechanism

Answer: What term is used to describe the sequence of elementary steps that together comprise an entire chemical reaction?

Question: Passage: Part of a cycle that holds an element or water for a long period of time is called a reservoir . The ocean is a reservoir for water. The deep ocean may hold water for thousands of years. Answer: a reservoir

Answer:
What is part of a cycle that holds an element or water for a long period of time called?