Answer:
because all green plants release oxygen and non- green plants do not release oxygen.
Explanation: One of the first uses of an isotopic tracer in biology was in the determination of the source of the oxygen produced by photosynthesis. Using a heavy isotope of oxygen, 18O, to label one of the two reactants in the photosynthetic process, Samuel Ruben and collegues at UC Berkeley were able to determine that the oxygen gas came not from carbon dioxide, but water.