Explain how carbon was trapped in fossil fuels formed from dead organisms.

Carbon was trapped in fossil fuels when dead organisms, such as plants and marine life, were buried under layers of sediment. Over millions of years, heat and pressure transformed these organic materials into fossil fuels like coal, oil, and natural gas, effectively locking the carbon away from the atmosphere.

A strong answer should directly address the approved learning objective to explain that some carbon was trapped in fossil fuels formed from dead organisms. This question belongs to How carbon dioxide decreased within The composition and evolution of the Earth's atmosphere, so the response should use that exact curriculum context rather than a generic statement. The answer is correct when it names the key idea, explains the link to fossil fuels, and keeps the wording specific to AQA GCSE revision.