Which of the following are reactants in photosynthesis?

• A. Water and Oxygen
• B. Water, Carbon Dioxide, and Light
• C. Carbon Dioxide and Light
• D. Water and Light

Answer: (B)

Photosynthesis uses light energy to drive the transformation of carbon dioxide and water into glucose and oxygen. The substances that start and are converted in this process are water and carbon dioxide, with light providing the energy that makes the reaction proceed. Oxygen ends up as a product that’s released, not a starting material. That’s why water and carbon dioxide are the key inputs, and light is included as a reactant in this context because it supplies the energy needed to convert them. The other options either include a product (oxygen) or miss the essential carbon dioxide or energy input.