The question I am having trouble with is:

Question

Scientists originally were uncertain whether the oxygen gas produced during photosynthesis came from CO2, H2O, or from both. How could radioisotope tracers be used to settle that question?

Answer 1

6CO2 + 6H2O ==> C6H12O6 + 6O2
Make O in CO2 radioactive; see where the radioactive O is at the end.
Make O in H2O radioactive; see where the radioactive O is at the end.