The chemical reaction for photosynthesis is

6COX2+12HX2O+light energyCX6HX12OX6+6OX2+6HX2O \ce{6CO2 + 12 H2O + light energy -> C6H12O6 + 6O2 + 6H2O}

If the input water is labeled with a radioactive isotope of oxygen, X18X2218O\ce{^{18}O}, then the oxygen gas released as the reaction proceeds is also labeled with X18X2218O\ce{^{18}O}. Which of the following is the most likely explanation?