Question

chemical reaction predictions quick check 5 of 5 why are iron oxides in rocks evidence for prehistoric photosynthesis? (1 point) iron oxides form when carbon dioxide, a reactant of photosynthesis, is in the atmosphere. iron oxides form when cyanobacteria use carbon dioxide for photosynthesis. iron oxides form when oxygen, a product of photosynthesis, is in the atmosphere. iron oxides form when cyanobacteria produce oxygen via photosynthesis.

Explanation:

Iron oxides (rust) form through oxidation reactions. Photosynthesis produces oxygen ($O_2$). When oxygen is present in the atmosphere (a result of photosynthetic organisms like cyanobacteria over time), it can react with iron in rocks to form iron oxides. Option A is incorrect as carbon dioxide ($CO_2$) is a reactant in photosynthesis but not directly related to iron oxide formation. Option B is wrong because it's the oxygen (not carbon dioxide use) that leads to iron oxide formation. Option D is too specific about cyanobacteria (while any photosynthetic organism producing oxygen could contribute) and the key is oxygen in the atmosphere (not just production by cyanobacteria) for iron oxide formation.

Answer:

C. Iron oxides form when oxygen, a product of photosynthesis, is in the atmosphere.