I disagree, from what I had learned.
The air enters our body through the nose and moves down the windpipe to the lungs. At the lungs, the oxygen enters the blood. The blood transports oxygen to the heart and then to the other parts of the body. At the same time, the blood transports carbon dioxide from the parts of the body to the heart and then to the lungs. Carbon dioxide is removed from our body when we breathe out.
What’s more. C can’t have more amount of oxygen then D.
“but C cannot have only a tiny bit lesser oxygen than A as the blood from the heart to the lungs has the least amount of oxygen and the blood from the lungs to heart has the most amount of oxygen”
This is not true for you do not know how much oxygen the graph represents. Therefore the answer is 1.