阅读下列材料，从每题所给的四个选项（A、B、C、D）中，选出最佳选项。
After that, we went on down the river. It was very dark that night and it wasn't easy to see where we were going. Suddenly, a big steamboat came at us very fast, and the next minute it was right over us. Jim and I jumped off the raft (木筏) into the water. The boat hit the raft and went on up the river.
When I came up out of the water, I couldn't see Jim anywhere. I called out his name again and again, but there was no answer. “He's dead!” I thought. Slowly, I swam to the side of the river and got out. I saw that I was near a big old wooden house. Suddenly a lot of very angry dogs jumped out at me. They made a terrible noise and someone called from the house, “Who's there?”
“George Jackson,” I answered quickly. “I've fallen off a river boat.”
Well, the people who lived in that house were very kind, and they took me in and gave me some new clothes and a good meal. I told them that my family were all dead, so they said I could stay with them as long as I wanted. It was a beautiful house and the food was good there, so I stayed.
A few days later one of the slaves in the house came to me and said, “Come with me!” Together, we went down to some trees by the river. “In there!” he said and went away.
On the ground, I found a man, asleep. It was Jim! I was really pleased to see him. When the steamboat hit the raft, Jim told me, the raft didn't break up. Jim swam after it and caught it. Then he began to look for me.
We decided to leave at once. It's all right living in a house for a little while, but you feel more free and easy and comfortable on a raft.
2CO2+4H2O，则X的化学式为（ ）