How do you prove that the source of oxygen in photosynthesis is the water molecule and not carbon dioxide?

Source of Oz in Photosynthesis :
(a) It was previously assumed that the source of oxygen in photosynthesis was C02 and not H20 molecules.
(b) According to recent studies oxygen is evolved from water and not from CO2.
© It was proved by the use of radioactive elements. For example, the water containing heavy oxygen (H2O18) was supplied to the leaves in place of natural oxygen, (H2O16).
(d) On observation it was found that oxygen evolved in photosynthesis was heavy oxygen 1802 and not the natural oxygen.
(e) Now, the heavy isotope of carbon dioxide (C14O2) was supplied to the plant.
(f) Thus it shows that in photosynthesis, oxygen
evolved from water molecules and not from carbon dioxide.