Aquatic Animals, in General, Face More of a Challenge When

Aquatic animals, in general, face more of a challenge when extracting oxygen from their environment than do terrestrial animals. Why?

A) Less oxygen is available in water than in air.
B) Less energy is required to ventilate gills than lungs.
C) Aquatic animals lose ions to the environment.
D) Gills have thinner cells than alveoli.
E) Parabronchi have twice the metabolic rate of spiracles.