Answer: A and B
Explanation: From the chemical equation of photosynthesis given above, six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen. From the chemical equation, 6CO2 = six molecules of carbon dioxide, 6H2O = six molecules of water, C6H12O6 = one molecule of glucose and 6O2 = six molecules of oxygen.
Therefore, the essential substances to photosynthesis are carbon dioxide (CO2) and water (H2O).