What process creates the sand dune

A dune is a mound of sand formed by the wind, usually along the beach or in a desert. Dunes form when wind blows sand into a sheltered area behind an obstacle. Dunes grow as grains of sand accumulate. Every dune has a windward side and a slipface.

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Explanation:

a. the production of carbon dioxide through the process of photosynthesis

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.