Answer:
Water (H2O)
Explanation:
Photosynthesis is a process that forms organic sugars such as glucose. It occurs during dark reactions. Sugars have atoms of carbon, oxygen, and hydrogen. For the purpose, atmospheric CO2 serves as a source of carbon while water molecules serve as a source of hydrogen atoms. When water molecules with radio-labeled hydrogen atoms are supplied to plants, the radioactivity can be seen in the glucose formed by the process of photosynthesis.