Yes Carbon dioxide is necessary for photosynthesis because Photosynthesis didn't go on because there was no Carbon dioxide in flask X.
The carbon dioxide was absorbed by the sodium hydroxide. In flask Y, the colour of the iodine solution turned blue black. This is because starch was present in the leaf of flask Y. Starch was present because it underwent photosynthesis.
Well they share the function of giving nutrition to the buds and the leaves on the plant , but they preform this function in different way from each other
The best answer i see is A)- Cooling
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