Question 4
Why can no laboratory experiment show that water is needed for photosynthesis?
The right answer is the one marked above.
Because a plant given no water dies, so the experiment would change more than one thing. A dead plant does not photosynthesise for reasons that have nothing to do with water being a raw material.
Scientists showed it a different way, with water containing the 18O isotope: the labelled oxygen turned up in the oxygen gas released.