Bottlenose dolphins breathe air but can sleep in the ocean ...

[Millan]

Bottlenose dolphins breathe air but can sleep in the ocean because
A) they cease breathing while sleeping and remain underwater.
B) they sleep for only 30 minutes at a time, which is the maximum interval they can cease breathing.
C) they fill their swim bladder with air to keep their blowholes above the surface of the water while they sleep.
D) they move to shallow water to sleep, so they do not need to swim to keep their blowholes above the surface of the water.
E) they alternate which half of their brains is asleep and which half is awake.

[Celeste]

E