They contain symbiotic bacteria called rhizobia within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants; this helps to fertilize the soil.
I would say A. Because it seems like a logical explanation.
Bacteriophage, also called phage or bacterial viruses, any of a group of viruses that infect bacteria.
