下列各种不属于生物的是（　　）

                                                                              A Trip to the Forest
   One day Bob took two of his friends into the mountains. They put up their tents and then rode off to a forest to see how the trees were growing.
   In the afternoon when they were about ten kilometres from their camp（营地）, it started to snow. More and more snow fell. Soon Bob could hardly see his hands before his face. He could not find the road. Bob knew there were two roads. One road went to the camp, and the other went to his house. But all was white snow. Everything was the same. How could he take his friends back to the camp?
   Bob had an idea. The horses! Let the horses take them back! But what would happen if the horses took the road to his house? That would be a trip of thirty-five kilometres in such cold weather!
   It was getting late. They rode on and on. At last the horses stopped. Where were they? None of them could tell. John looked around. What was that under the tree? It was one of their tents!