Why Is Gas Exchange in an Aquatic Environment More Challenging

Why is gas exchange in an aquatic environment more challenging than gas exchange in the air?

A) The large surface area of gills makes them more susceptible to drying out.
B) Less oxygen is available in water than in air.
C) Carbon dioxide is eliminated by active transport in an aquatic environment.
D) The countercurrent exchange process limits the amount of oxygen that can be extracted from water.
E) Gills have a poor blood supply.