Passage One

A．nimals have different ways of protecting themselves against wintertime weather. Some animals grow heavy coats of fur or feathers, while others dig into the ground to find a warm wintertime home.

Some animals spend the winter in a deep sleep because by going to sleep they avoid the time of the year when food is scarce and the temperatures are low. Their sleep is known as hibernation.

There is much about hibernation that puzzles scientists. For example, they are wondering how hibernation came into being. Some scientists have explored the possibility that animals release a chemical that starts them hibernating.

One thing that scientists are certain about is that animals hibernate only when it is cold. Hibernation is a seasonal practice.

Some animals that fall into a wintertime sleep are not true hibernators because they spend only a part of the cold season asleep. Bears, for example, can easily be awakened from their winter nap. They are not true hibernators.

Sometimes it is difficult to determine whether a particular animal is a true hibernator. For example, some mice hibernate, but others do not. The same is true of bats. Some of them hibernate. Others do not.

36. Hibernation is a seasonal practice. This means it ______.

A． takes place only during a particular season

B． occurs only during the night

C． is a daily practice

D． only happens when a species becomes over-populated