A botanist performs a series of experiments in which she radioactively labeled different possible sources of hydrogen available for use by plants. When she later examines photosynthetically derived compounds from the plants, she will find that their hydrogen came from

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.

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i am going to guess that it is co2.