It has been hypothesized that fungi and plants have a mutualistic relationship because plants make sugars available for the fungi's use. What is the best evidence in support of this hypothesis?

A) Radioactive label experiments show that plants pass crucial raw materials to the fungus for manufacturing sugars.
B) Fungi associated with plants have the ability to undergo photosynthesis and produce their own sugars, while those not associated with plants do not produce their own sugars.
C) Radioactively labeled sugars produced by plants eventually show up in the fungi they are associated with.
D) Fungi survive better when they are associated with plants.