How is carbonic acid formed from an oxide and water?

• A. By reacting carbon dioxide with water: $C_0{O}_2+H_{2}O\to H_2{C}_0{O}_3$
• B. By reacting sodium carbonate with hydrochloric acid: $N_{\mathrm{a}}{C}_0{O}_3+H_{2}C{l}_2\to H_2{C}_0{O}_3+2NaCl$
• C. By reacting elemental carbon with water: $C+H_{2}O\to H_2{C}_0{O}_3$
• D. By reacting carbon monoxide with water: $CO+H_{2}O\to H_2{C}_0{O}_3$

Answer: (A)