Gas Exchange Is More Difficult for Aquatic Animals with Gills

Gas exchange is more difficult for aquatic animals with gills than for terrestrial animals with lungs because

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 collapse in air.