The answer is that it means that the oxygen would be difficult to carry throughout the body, which happens that ATP would be lacking. Because to create energy there’s a need for respiration and it requires an oxygen. Also, other body systems will slow down because anaerobic respiration doesn't create enough ATP for the entire body.
The answer is A.photosynthesis
D because if the species was to be removed from an ecosystem, everything that needed that species as food, or the species ould have to adapt to the foods available in the new ecosystem they live in. Hope I helped, and let me know if you are still not 100% sure about the answer, and I'll try to clarify what I said.