Why do plants containing nitrogen-fixing bacteria in their roots need less nitrogen-based fertilizer than others?

A) The plants break down the bacteria as a source of nitrogen-rich nutrients.
B) The bacteria convert nitrogen in the air into a form that can be taken up by the plants.
C) The plants and bacteria work together to attract birds, which deposit nitrogen-rich droppings near the plant.
D) The bacteria kill and digest earthworms and other animals in the soil, providing excess nitrogen to the plants.