Question 3
Three of the four factors were proved necessary by removing them and testing for starch. Which was proved a different way, and how?
The right answer is the one marked above.
Water, using water containing the 18O isotope. A plant deprived of water dies, so subtraction was not available. Instead the water was labelled and followed.
The other three are exactly the three subtraction experiments: black polythene for light, KOH for carbon dioxide, and a variegated leaf where nature had removed the chlorophyll already.