Why Do Larger Animals Depend on an Internal Transport System

Multiple Choice

Why do larger animals depend on an internal transport system (like the circulatory system in mammals) to distribute oxygen when small organisms get along fine without one?

A) Diffusion does not occur in organisms with more than about 300 cells.
B) Large organisms have larger cells and use more oxygen per cell than the cells of small organisms;diffusion alone is unable to supply the extra oxygen needed.
C) Larger animals lose oxygen to the atmosphere as it diffuses through their skin;an internal transport system is needed to replenish these losses.
D) Diffusion delivers oxygen very slowly over distances larger than a typical cell's diameter;transport systems move oxygen close to the cell so that the diffusion distance remains small.