单选题 We know that many animals do not stay in one place. Birds, fish and other animals move from one place to another at a certain time. They move for different reasons; most of them move to find food more easily, but others move to get away from place that are too crowded.
When cold weather comes, many birds move to warmer places to find food. Some fishes give birth in warm water and move to cold water to feed. The most famous migration (迁移) is probably the migration of the fish, which is called "salmon". This fish is born in fresh water but it travels many miles to salt water. There it spends its life. When it is cold, it returns to its birthplace in fresh water. Then it gives birth and dies there. In northern Europe, there is a kind of mouse. They leave their mountain homes when they become too crowded. They move down to the low land. Sometimes they move to the seaside and many of them are killed when they fall into the sea.
Recently, scientists have studied the migration of a kind of lobster (龙虾). Every year, when the season of the bad weather arrives, the lobsters get into a long line and start to walk across the floor of the ocean. Nobody knows why they do this, and nobody knows where they go.
So, sometimes we know why humans and animals move from one place to another, but at other times we don"t. Maybe living things just like to travel.

【正确答案】 B

【答案解析】[解析] 推理推断题。根据文章第二段倒数第三、四句“In northern Europe, there is a kind of mouse. They leave their mountain homes when they become too crowded.”中的crowded可知，当居住的地方没有足够的空间时，老鼠就开始迁徒了。