Many species of plants have nodes that are filled with nitrogen-fixing bacteria on their roots. The nitrogen that the bacteria "fix" is then used by the plant to build plant tissues. Where does most of the nitrogen that the bacteria use come from? A. The nitrogen is secreted by the plants through their root systems. B. The nitrogen is absorbed into the soil from the atmosphere. C. The nitrogen is formed by chemical reactions taking place between different minerals in the soil. D. The nitrogen is produced by the node's outer membrane.