阅读理解。 Growing kids need sleep, as do people of all ages. Indeed, research shows that health and safety both suffer when we try to get by with too little shuteye. So it's fortunate that our bodies do such a good job of waming us when it's time to hit the sack . Like humans, other animals also take time out to rest. You've probably seen a lion dozing at the zoo, or maybe watched our dog snooze away lying in its bed. In fact, sleep is a necessity for every animal that's ever been studied. How long animals sleep, though, varies widely. Elephants and giraffes sleep only about 2 to 4 hours a day, while bats may nod off for up to 20 hours. Ducks sleep together, and the birds on the edge of the group sleep with the outside eye open and half of their brain awake-probably to keep watch while the other half of their brain snoozes. Some birds may even sleep while flying. The fact that all animals make time for sleeping, even under very dangerous situations, suggests that sleep is very important. And indeed, some facts suggest that sleep is important for leaming and forming permanent memories. But sleep may also be primarily a way for animals to save energy and stay out of harm. This may help explain why meat-eating creatures sleep more than herbivores, which are animals that live only on plants. Herbivores, like cows and zebras, need to spend more time searching for and grazing on food than meat eaters do, such as lions and other big cats. So a big cat might be better off sleeping for a while after it eats, rather than prowling around and risking injury. But that's just a guess. Scientists need to study other animals as well before they can fully understand the benefits (好处) of sleep and know which benefits are most important for a particular species (物种). One thing is certain: enough sleep is important to health and learning. So give in when a strong wish to sleep hits, and catch plenty of ZZZs. 1. The underlined phrase 'to hit the sack' means ____. A. to eat something B. to drink something C. to get up D. to go to sleep 2. From the last paragraph, we can infer ____. A. enough sleep may improve your learning in class B. if you want to sleep in class, fall asleep immediately C. you are easy to give in if you have plenty of sleep D. if you have a strong wish to sleep, you are perhaps ill 3. The main idea of the passage is about ____. A. all people need sleep B. how animals sleep in the open air C. why animals need sleep D. when animals feel sleepy