Concentrated sulfuric acid is passivated by iron and aluminum at room temperature, which reflects the strong oxidation of concentrated sulfuric acid, so the reaction between iron and concentrated sulfuric acid at room temperature is difficult. A is wrong for C. The blackening phenomenon of sucrose after adding concentrated sulfuric acid is that concentrated sulfuric acid dehydrates and carbonizes sucrose. The gas that smells sour smell is that concentrated sulfuric acid oxidizes carbon into carbon dioxide, and carbon dioxide itself is reduced to sulfur dioxide, which shows that concentrated sulfuric acid dehydrates and oxidizes, and B is correct. In the industrial production of sulfuric acid, it is correct to use 98.3% sulfuric acid instead of H 2 O or dilute sulfuric acid to absorb SO 3 to prevent the formation of acid mist. So choose C.