The Bottlenose Dolphin Breathes Air but Can Sleep in the Ocean

The bottlenose dolphin breathes air but can sleep in the ocean because it

A) ceases breathing while sleeping and remains underwater.
B) sleeps for only 30 minutes at a time,the maximum interval for which it can cease breathing.
C) fills its swim bladder with air to keep its blowhole above the surface of the water while it sleeps.
D) moves to shallow water to sleep,so it does not need to swim to keep its blowhole above the surface of the water.
E) alternates which half of its brain is asleep and which half is awake.