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 places 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 old, it returns to its birthplace in fresh water. Then it gives birth and dies there. In northern Europe, there is a kind of mice. They leave their mountain homes when they become too crowded. They move down to the low land. Sometimes they move all the way 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 lobsters（龙虾）. Every year, when the season of 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.
51. Most animals move from one place to another at a certain time to __________ .
A. give birth B. enjoy warmer weather
C. find food more easily D. find beautiful places
52. The fish called “salmon” spends a long life in __________ .
A. salt water B. rivers
C. fresh water D. its birthplace
53. What is the main idea of the passage?
A. Animals move in order to find food more easily.
B. The migration of the fish called “salmon” is the most famous migration.
C. Living things move from one place to another because they like to travel.
D. Sometimes we know why and how living things move from one place to another, but sometimes we don’t.