Given a scientific passage and an answer, generate a question for the given answer.
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Question: Passage: Oxygen is needed to make ozone. Ozone is a molecule made of three oxygen ions. Ozone in the upper atmosphere blocks harmful solar radiation from reaching the surface. Without oxygen, life on Earth would have been very simple. Answer: three

Answer: How many oxygen ions make up an ozone molecule?

Question: Passage: Humans learn behaviors through conditioning, as well. A young child might learn to put away his toys by being rewarded with a bedtime story. An older child might learn to study for tests in school by being rewarded with better grades. Can you think of behaviors you have learned by being rewarded for them?. Answer: conditioning

Answer: How do humans learn behaviors?

Question: Passage: Quaternary structure is a larger assembly of several polypeptide chains, now referred to as subunits of the protein. The quaternary structure is stabilized by the same interactions as the tertiary structure. Complexes of two or more polypeptides are called multimers. Specifically, a dimer contains two subunits, a trimer contains three subunits, and a tetramer contains four subunits. Answer: quaternary structure

Answer:
What structure is a larger assembly of several polypeptide chains that are now referred to as subunits of the protein?