Answer:
Sleep is a difficult problem for dolphins because dolphins __________.
Step-by-step explanation:
Sleep of unihemispheric slow waves: When the dolphins rest, only one hemisphere of their brain loses consciousness, so they only close one eye, the left eye closes when the right half of the brain sleeps, and vice versa. Meanwhile, the remaining cerebral hemisphere is responsible for monitoring breathing.
The answer is: Because dolphins only close one eye when sleeping.