Why Do Humans Rely on a Circulatory System to Distribute

Why do humans rely on a circulatory system to distribute oxygen?

A) Diffusion does not occur in organisms that have more than about 300 cells.
B) Humans do not need as much oxygen as small organisms do, so we use inefficient transport systems like the circulatory system rather than diffusion.
C) Humans need an internal transport system in order to quickly replace oxygen that is lost because of our high surface area of skin.
D) Diffusion is very slow over all but the shortest distances, so oxygen must be delivered to our cells to speed up the process.