Final answer:
After the light is switched off, the amount of radioactively labeled glucose decreases because glucose is converted into carbon dioxide and water through the process of cellular respiration.
Step-by-step explanation:
After the light is switched off in the experiment, the amount of radioactively labeled glucose decreases because glucose is converted into carbon dioxide and water (option A). During the process of cellular respiration, glucose is broken down in the cells to release energy. This process involves the complete oxidation of glucose, resulting in the production of carbon dioxide and water as byproducts.