Greatest diffusion should occur where the concentration gradient is greatest: in the countercurrent system at the ends of the system, though the concentration difference and the absorption rate are nearly constant along its length; in the concurrent system, at its start when the difference is 100%.
Work Step by Step
To understand this, see that the gradient in oxygen concentration is most critical to drive diffusion. Then look at Fig. 48.6 to find where this is for each kind of system.