A lot of animals travel from one place to another． We call this migration．

    In Africa， large animals， like elephants and zebras， migrate to find food and water． They usually follow the same ways every year．

    A lot of birds migrate to find food and better weather， too． They are usually birds that eat insects．They spend the summer in northern Europe， because there are lots of insects there．In the winter there aren't any insects， so the birds fly south to southern Europe and Africa，

    Some insects migrate， too． In North America， millions of monarch butterflies fly south to spend the winter in Mexico， California and Florida， where it's warmer． They travel 50﹣65 km each day and they travel about 1，125 km，

    Some fish migrate to breed（繁殖）．Salmon（鲑鱼） can swim over 20，000 km in their life．They are born in rivers in Ireland， Scotland and other places in northern Europe． The young fish swim down the river to the sea and into the Atlantic Ocean．They live in the ocean until they are adults．Then they return to the river where they were born， They lay their eggs in the river and then they usually die． Salmon do this， because their eggs are safer in the river， Other fish can't eat them．

    Arctic terns （北极燕鸥） travel the furthest when they migrate． They spend the summer in the Arctic，but when winter comes they fly to the Antarctic， because it's summer there． The next year they fly back to the Arctic again．In one year these small birds travel 36 ，000km from one end of the earth to the other and back again． Nobody knows how they do it．

（1）According to the passage， monarch butterflies in North America migrate to　　．

A． find insects

B． look for water

C． find better weather

D． produce young

（2）Adult salmon swim back to the river because　　．

A． they won't be eaten by other big fish

B． they want to lay eggs in a safer place

C． they are getting ready to die there

D． they want to go where they grew up

（3）Which of the following is true about animals' migration？　　

A． Elephants in Africa usually travel the same way each year．

B． Most birds fly to southern Europe and Africa in summer．

C． Some insects travel 1，125 km each day．

D． Salmon swim 20 ，000 km each year．

（4）The example of Arctic terns in the last paragraph is used mainly to show　　．

A． the distance of migration

B． the purpose of migration

C． the direction of migration

D． the time of migration