Cellular Energetics Question 1

The chemical reaction for photosynthesis is

$$ \ce{6CO2 + 12 H2O + light energy -> C6H12O6 + 6O2 + 6H2O} $$

If the input water is labeled with a radioactive isotope of oxygen, $\ce{^{18}O}$, then the oxygen gas released as the reaction proceeds is also labeled with $\ce{^{18}O}$. Which of the following is the most likely explanation?

During the light reactions of photosynthesis, water is split, the hydrogen atoms combine with the $\ce{CO2}$, and oxygen gas is released.

During the light reactions of photosynthesis, water is split, removing electrons and protons, and oxygen gas is released.

During the Calvin cycle, water is split, regenerating NADPH from NADP⁺, and oxygen gas is released.

During the Calvin cycle, water is split, the hydrogen atoms are added to intermediates of sugar synthesis, and oxygen gas is released.