What are the two main functions of photosynthesis?
(A) harvests light energy for cellular energy
(B) returns oxygen into the atmosphere
(C) fixates carbon
(D) breaks down water molecules
(A) and (C) are correct. Photosynthesis harvests energy for ATP and fixates carbon for food. It does return oxygen into the atmosphere (B) and break down water molecules (D), but these are not its main functions. It is the need for energy and food that perpetuates this cycle.