It has been hypothesized that fungi and plants have a mutualistic relationship because fungi provide critical nitrogen for the plants' use. How do we know this happens?

A) When plants are associated with fungi, they can fix atmospheric nitrogen that has been tagged with a radioactive label.
B) Plants acquire more radioactive nitrogen when they are associated with fungi.
C) Radioactively labeled sugars in plants eventually find their way to their symbiotic fungi.
D) Radioactively labeled nitrogen shows up in fungi when they are symbiotic with plants.