We know that many animals do not stay in oIle place.Birds,fish and other animals move fromone place to another at a.certain time

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问题     We know that many animals do not stay in oIle place.Birds,fish and other animals move fromone 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 warn’1 water and move to cold water to feed.The most famous migration is probably themigration 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 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 lobster.Every year,when the  season of bad weather arrives,the lobsters get into a long time 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.
Mice in northern Europe go away from homes when__________.

选项 A、they are short of food
B、there is not enough space
C、it’s getting warmer in the low land
D、it’s getting colder in the mountains

答案B

解析 根据“In northern Europe,there is a kind of mouse.They leave their mountain homes when theybecome too crowded.”中的crowded可知,住宅中已经没有足够的地方供老鼠生存了。
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