Why Is Gas Exchange More Difficult for Aquatic Animals with Gills

Multiple Choice

Why is gas exchange more difficult for aquatic animals with gills than for terrestrial animals with lungs?

A) Water is less dense than air.
B) Water contains much less O2 than air per unit volume.
C) Gills have less surface area than lungs.
D) Gills allow only unidirectional transport
E) Gills allow water to flow in one direction