Vernon is using the diagram shown below to develop a model of photosynthesis. The letters A, B, and C each represent a reactant or product of the process. In the light-dependent reactions, light strikes the oval thylakoids. Compound A enters the thylakoid, and compound B leaves it. In the light-independent reactions, compound C enters a cycle in the stroma, and sugars leave the cycle. ATP and NADPH cycle between the two sets of reactions.

What is the role of compound C?

A. a reactant that provides the energy for assembling sugars
B. a reactant that provides the carbon and hydrogen for sugars
C. a reactant that provides the carbon and oxygen for sugars
D. a product that forms along with sugars