题干

设A、B两镇相距x千米,甲从A镇、乙从B镇同时出发,相向而行,甲、乙行驶的速度分别为u千米/小时、v千米/小时,并有:

①出发后30分钟相遇;

②甲到B镇后立即返回,追上乙时又经过了30分钟;

③当甲追上乙时他俩离A镇还有4千米.求x、u、v.

根据题意,由条件③,有四位同学各得到第3个方程如下,其中错误的一个是(   )

A:x=u+4

B:x=v+4

C:2x﹣u=4

D:x﹣v=4

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A

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    Zhalong is a nature reserve in Heilongjiang in northeast China.It is one of the world's most important wetlands.The area provides food and shelter for wildlife.It is an ideal home for different kinds of plants,fish and birds.

    Many birds live comfortably in Zhalong Nature Reserve all year round,while some only stay for a short time.There are many fish in the wetlands,and the birds can easily catch them for food.Zhalong Nature Reserve is an important living area for the rare red-crowned cranes.There are not manyred-crowned cranes in the world.Some of them live in Zhalong.

    Some people want to change the wetlands to make more space for farms and buildings.This means that there will be less and less space for wildlife.

    More and more birds are in danger because they do not have enough living space.Many of them died.The Chinese government wants to protect these endangered birds,and they can be safe in Zhalong.

    Every year a lot of tourists go to Zhalong to watch birds.This year members of ourBird watching Club are going to study the different kinds of birds in Zhalongand the changes in their numbers.The study begins next month.

    We do a bird count once a year.We are now inviting secondary school students to help.We need more people to help us count and do something to help the birds.Many people do not understand the importance of the wetlands.We hope this information will help them understand and make them actively take action to protect wildlife.