阅读下列短文，从每题所给的四个选项（A、B、C和D）中，选出最佳选项。

　　Monarch butterflies（黑脉金斑蝶）are a common summer sight in the northern United States and Canada．These large orange and black insects（昆虫）brighten parks and gardens asthey fly lightly among the flowers．What makes monarchs particularly interesting is that they migrate（迁飞） — all the way to California or Mexico and back．They are thought to be the only insect that does this．

　　Every year in the late summer monarchs begin their journey to the south．Those heading for Mexico go first for the Louisiana-Mississippi area,then fly across the Gulf of Mexico into Texas．Once in Mexico,they settle themselves in one of about fifteen places in a mountain forest filled with fir trees．Each place provides a winter home for millions of monarchs．The butterflies are so many that they often cover entire trees．When spring comes,they begin their long journey north．

　　The question is often asked whether every butterfly makes the round-trip journey every year．And the answer is no．The average monarch lives about nine months．So one flying north might lay eggs in Louisiana and then die．The eggs of that generation may be found in Kentucky;the eggs of the next generation may end up in Wisconsin or Michigan．The last generation of the season,about the fourth,will make their way back to Mexico and restart the journey．

　　Scientists learn about monarchs' migration by catching and making marks on the insects．By recatching a monarch with such a mark and noticing where it came from,the next scientist can get to know things like the butterfly's age and its routing（路线）．

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阅读下列短文，从每题所给的四个选项(A、B、C和D)中，选出最佳选项。

　　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 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(龙虾)．Every year, when the season of bad weather arrives, the lobster 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 travel．

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