Monarch butterflies from eastern Canada make the most amazing journey in the insect world. Each year, this butterfly travels about 3000miles to its winter home in central Mexico(墨西哥). How can it fly so far? And why does it make this long and dangerous trip? Scientists still don't have an answer.

 For many years, people in Mexico wondered where the orange-and-black butterflies came from every winter. Then, in 1937, a scientist started to follow and study the butterflies. For the next 20 years, he discovered that one butterfly started its journey in Canada. Four months later, it arrived in Mexico.

 The length of the butterflies find their way back to the same place? Another amazing thing is that the butterflies always return to the same area in central Mexico.

 How do the butterflies find their way back to the same place? This is an interesting question because only every fourth generation(代) makes the trip south. In other words, the butterfly that travels to Mexico this year is the great-great-grandchild of the butterfly that traveled there last year.

 Each year, four generations of a Monarch butterfly family are born. Each generation of the family has a very different life. The first generation is born in the south in late April. It slowly moves north, reproduces(繁衍), and then dies. On the trip north,two more generations are born, reproduce, and die. Each of these generations of butterflies is born. This generation has a much longer life. It lives for about eight months. This generation of butterflies makes the amazing journey back to the winter home of its great-great-grandparents. The butterflies spend the winter there, and in the spring they reproduce and then die. Their offspring will be the first generation of the next circle of life.

 Today, people are still studying the Monarch butterfly. But they are not clear about everything.

   We all need a healthy environment, but we produce waste every day and it does harm to our environment. Though we are young, we can still do something to help. In fact, even the simplest everyday activities can make a real difference to the environment. Here are some ideas for you to live a green life.
   Remember these three words: reduce, reuse and recycle. Reduce means “use less”. Don't waste things. This saves money and reduces pollution and waste going into the environment. Before we buy something new, think if it is really necessary or maybe the old one will be just as good! When we do buy things, choose local products if possible, and try not to buy too many things from abroad.
   Reuse means “use again”. Use things for as long as possible. When we buy things, make sure that they last a long time. We should take care of them so that they will last, and we should repair them if we can instead of throwing them away and buying new ones. Don't use a paper cup or a paper bag. It's better to use a china cup and a lunch box because you can use them again.
   Recycle means “change things into something else”. Although it takes energy to change something into something else, it's better than throwing things away or burning them. Find out what can be recycled in your neighbourhood and take part in recycling programmes. We should also buy products made from recycled materials, such as recycled paper, to help save trees.
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根据短文理解，完成下列各题。
    In every school there is "top" crowd that sets the pace, while the others follow their lead. For example, if the crowd decides that it is smart to wear bright red sweaters, then very soon everybody is wearing bright red sweaters.
    There is nothing wrong with that, except the fact that for some people bright red sweater is really unsuitable.
    The situation can even become dangerous, if the top crowd decides that it is smart to drink and to drive cars at seventy miles an hour. Then the people who follow the lead are endangering their lives. They are like the sheep being led to the slaughter(屠杀).
    Now it's possible that you have come across situations like these more than once in your life. In fact you probably did something you knew to be wrong one time or another. You may have excused yourself by saying, "Gee, the crowd does it."Well, let the crowd do it, but don't do it yourself. Learn to say "No".
    Develop your own standards(标准)and your own judgment. If you know the crowd is planning something you disagree to, you'd better take the courage to bow out in a polite way. You'll have the satisfaction of standing on your own two feet.

    In most parts of the world, many students help their schools make less pollution. They join environment clubs. In an environment club, people work together to make our environment clean.

    Here are some things students often do.

    No-rubbish lunches. How many lunch bags do you throw away after lunch? Environment clubs ask students to bring their lunches in bags that can be used again. Every week they will choose which classes make the least rubbish and report them to the whole school.

    No-car day. On a no-car day, nobody comes to school in a car including the students and the teachers. Cars give pollution to us, so remember:

    Walk, jump, bike, or run!

    Use your legs! It's lots of fun!

    Turn off the water! Do you know that some toilets can waste twenty to forty cubic (立方) meters of water an hour? In a year, that would fill in a small river. In the environment clubs, students repair those broken toilets.

    We love our environment. Let's work together to make it better.

    Now satellites(卫星)are helping to forecast(预报)the weather. They are in space and they can reach any part of the world.The satellites take pictures of the atmosphere(大气), because this is where the weather forms(形成). They send these pictures to the weather station. So meteorologists(气象专家)can see the weather of any part of the world. From the pictures, the scientists can often say how the weather will change.

    Today, nearly five hundred weather stations in sixty countries receive satellite pictures. When they receive new pictures, the meteor- ologists compare(比较)them with earlier ones. Perhaps they may find that the clouds have changed during the last few hours .This may mean the weather on the ground may soon change, too. In their next weather forecast, the meteorologists can say this.

    So the weather satellites are a great help to the meteorologists. Before satellites were invented, the scientists could forecast the weather for about 24 or 48 hours. Now they can make good forecasts for 3 or 5 days. Soon, perhaps , they may be able to forecast the weather for a week or more ahead(提前)