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.

Vernon wonders whether NADPH and ATP should be classified as reactants or as products of photosynthesis. How should these compounds be classified?

A. as reactants, because they are used up by photosynthesis
B. as products, because they are generated by photosynthesis
C. as either reactants or products, depending on the rate that photosynthesis occurs
D. as neither reactant nor product, because they are recycled instead of made or used up