I think the answer is either B or C, hope this helps! Sorry if it doesn't!
Answer:
The diagram can be improved by:
Lungs
↓
oxygen
↓
Red blood cells (carrying oxygen)
↓
Organs (like stomach and liver etc) from where carbon
is taken and oxygen is supplied
↓
RBC's carrying Carbon dioxide to the lungs
The component which is missing in the diagram are the organs where exchange of gases occurs. The red blood cells carry oxygenated blood from the lungs to all parts of the body and carries the wast carbon dioxide gas from them back to the lungs. The carbon dioxide is then exhaled by the lungs.
Density dependent would be the answer
