With parallel flow, diffusion of urea happened at regions I and II but not at regions III, IV, or V because... A
Urea was too big to fit through the pores in regions III, IV, or V.
B
There was no concentration gradient for urea in regions III, IV, or V.
С
There was no urea in the blood in regions III, IV, and V.
D
The pore size was too small in regions III, IV, and V.