Explain why limestone is considered a long-term carbon store.

Limestone is considered a long-term carbon store because it is formed from the accumulation of carbonate sediments, which contain carbon. Over time, these sediments can trap carbon, preventing it from re-entering the atmosphere.

A strong answer should directly address the approved learning objective to describe limestone, coal, crude oil and natural gas as long-term carbon stores. 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 limestone, and keeps the wording specific to AQA GCSE revision.