Why are plants necessary in a biodome

This natural fertilizer provides rich nutrients for the plants. The plant roots strip the nutrients from the water as it trickles through the soil and back down to the ponds, arriving clean and full of oxygen for the fish. The Biodome is very inexpensive to operate and easy to maintain. A biodome is a form of controlled, self-sufficient eco-system that closely replicates the natural outdoor environment. It is a scientific based form of a greenhouse that can made up of various geodesic spheres, although it's not always the case.

The correct answer is b