Large number of capillaries to maintain a diffusion gradient.
Thin epithelium provides a short diffusion pathway
aii) It maintains a concentration gradient so diffusion occurs across the whole length of the gill.
b) Less energy is required.
ci) Larger surface area for gas exchange.
cii) Lowers water potential of tissues therefore liquid moves in by osmosis.
ciii) Diffusion pathway is too long therefore won’t be able to supply enough oxygen.
I hope that has helped !!