Why Do Many Large Animals Rely on Internal Transport Systems

Why do many large animals rely on internal transport systems (like the circulatory system) to distribute oxygen?

A) Diffusion does not occur in organisms with more than about 300 cells.
B) Large organisms do not need as much oxygen as small organisms so they utilize inefficient transport systems like the circulatory system rather than diffusion.
C) The internal transport system is needed to quickly replace oxygen lost due to the high surface area of the skin.
D) Diffusion is very slow over all but the shortest distances, so oxygen must be delivered to cells to speed up the process.