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

Passage: When ancient plants underwent photosynthesis, they changed energy in sunlight to stored chemical energy in food. The plants used the food and so did the organisms that ate the plants. After the plants and other organisms died, their remains gradually changed to fossil fuels as they were covered and compressed by layers of sediments. Petroleum and natural gas formed from ocean organisms and are found together. Coal formed from giant tree ferns and other swamp plants. Answer: chloroplasts
Output:
During photosynthesis what organelle is used by plants to change sunlight into chemical energy?