Almost all animals need to sleep to stay healthy．But sleeping can also be dangerous because other animals may hurt those who are sleeping．This leads to many different ways of sleeping that help them stay safe．Some of these ways may surprise you!

     Almost all bats sleep upside down．They hang from a branch of a tree or the ceiling of a cave．Up high，bats are safe from most other animals．If they are in danger，they just fly away．

     A dolphin sleeps with just half its brain at a time．It must stay awake to breathe and to watch for danger．Dolphin mothers and babies can go without sleep for as long as two months to stay safe．

     Some ducks sleep with just half their brains，too．These ducks often sleep in a line．The ducks at both ends help to keep the group safe．They keep one eye open to watch for danger．The ducks in the middle sleep deeply with both eyes closed．

     Seals breathe air，but that doesn't stop some of them from sleeping underwater．They hold their breath and dive deep to sleep for up to twenty minutes at a time．Sleeping deep underwater helps these seals stay safe from animals that hurt them near the surface．

     Horses often sleep standing up．Their legs lock in please so they don't fall over．If a horse is lying down，it needs a long time to get up．A horse can run from danger more quickly if it is already standing．

      A flamingo can sleep while standing on one leg．The standing leg locks so it stays straight．The other leg remains close to the flamingo's body．Its head rests on its back．A flamingo lives （and sleeps）near many other birds to stay safe．

     Some seabirds can sleep while they fly．These large birds spend most of their lives over the ocean．They are in danger on the water，but they are safe in the air．They fly for many hours without flapping（拍打）their wings．Sometimes they sleep while they are flying．

38．The animals that sleep with half their brains are　　

A．ducks and seals          

B．dolphins and ducks

C．bats and dolphins         

D．flamingos and seabirds

39．Horses often stand up when they sleep because　　

A．they can run from danger quickly   

B．they are afraid of falling over

C．this helps them to sleep deeply     

D．it takes them a long time to go to sleep

40．The underlined word"hunt"in Paragraph 5 probably means　　

A．offer to help

B．come to teach

C．refuse to protect  

D．want to kill

41．Which of the following can be the best title for the passage？　　

A．How animals sleep        

B．Sleep to stay healthy

C．Sleep an animals do       

D．What sleeping animals are like．