单选题   Imagine going to sleep in October and waking up in May! Well, marmots and ground squirrels stay warm by sleeping all winter. All this time, they do not wake up once. This special kind of sleep is called hibernation. During this sleep, the heart slows down, and the animal breathes more slowly, it doesn't move around, so it uses less energy.
    Animals like the marmot and the ground squirrel inhabit the coldest parts of the world. They need special talents to survive in these frigid places. Their furry coats keep them snug when the temperature falls below zero. It often gets this cold in the Arctic, a land that is just below the North Pole.
    Before the long winter, some animals eat and eat. After a while, they grow very fat. When the winter comes, they live on the fat saved up in their bodies. Layers of fat keep an animal warm.
    Arctic animals also have other ways to beat the cold. Rabbits in the Arctic, for example, have very small ears. Small ears keep heat in, while big ears let it out. Small things usually keep heat in. Have you ever slept in a room that is very small, and noticed how hot it can get?
    It rarely gets warm in the Arctic. But although summer seasons there are very short, the sun shines brightly. Plants seem to spring up before your eyes! Animals such as caribou look forward all year to summer, when they can eat fresh grass again. Every minute of sunshine is important to their lives.

单选题     A good title for this passage would be ______.

单选题     The way for marmot to keep warm is ______.

单选题     After reading this passage, we can guess that when an animal moves around, it ______.

单选题     To help the reader understand that small things keep heat in, the writer uses ______.