Raoul ate a meal of oats before seeing his doctor. Oats are made of mostly starch molecules. Raoul feels tired, so his doctor gave him a test and found that Raoul’s cells contained glucose molecules, but they did not contain enough oxygen molecules. Does this explain why Raoul feels tired? A. No, something else must be wrong because his cells have all the molecules they need.

B. Yes, his digestive system is breaking down starch molecules, but his respiratory system may not be working properly.

C. Yes, his respiratory system is working, but his digestive system is not getting energy from combining glucose molecules.

D. Yes, his digestive system is working, but his respiratory system is not breaking down oxygen, so his body stopped making energy.