Use the Following Information to Answer the Question(s) Below

Use the following information to answer the question(s) below.
For several decades now, amphibian species worldwide have been in decline. A significant proportion of the decline seems to be due to the spread of the chytrid fungus, Batrachochytrium dendrobatidis (Bd) . Chytrid sporangia reside within the epidermal cells of infected animals, animals that consequently show areas of sloughed skin. They can also be lethargic, which is expressed through failure to hide and failure to flee. The infection cycle typically takes 4 to 5 days, at the end of which zoospores are released from sporangia into the environment. In some amphibian species, mortality rates approach 100 percent; other species seem able to survive the infection.
-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? In experiments using radioactively labeled ________.

A) nitrogen, plants acquired more radioactive nitrogen when they were associated with fungi
B) nitrogen, labeled nitrogen showed up in fungi when the fungi were symbiotic with plants
C) nitrogen, when plants were associated with fungi, the plants could fix labeled nitrogen
D) proteins, plants transported labeled proteins to adjacent fungi