Given a scientific passage and an answer, generate a question for the given answer.
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Question: Passage: Carbon moves through ecosystems in two cycles that overlap. In the biotic cycle, it moves between living things and the air. In the abiotic cycle, it moves between the air, ground, and oceans. By burning fossil fuels, humans have increased the amount of carbon dioxide in the air. Answer: biotic cycle

Answer: In what part of the carbon cycle does carbon move between living things and the air?

Question: Passage: Neurons that innervate the muscles of the respiratory system are responsible for controlling and regulating pulmonary ventilation. The major brain centers involved in pulmonary ventilation are the medulla oblongata and the pontine respiratory group (Figure 22.20). Answer: neurons

Answer: What basic structures of the nervous system innervate the muscles of the respiratory system and are responsible for controlling and regulating pulmonary ventilation?

Question: Passage: As you age, you might start noticing pain in your knees or elbows. These are examples of joints. Joints are the part of the skeletal system that connect your bones. Joint pain is a common problem as people age. Answer: joints

Answer:
Knees and elbows are examples of what part of the skeletal system?