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下列各项中的两种物质既不是同分异构体，也不是同系物的是(  )

    How are you getting along with your high school? It may be hard for you as this is the first time you ____ (leave) your parents. I know that you are now having trouble ____ (communicate) with others. That's ____ you are new here and people don't know about you. But I think you can follow the advice below.

    First, you should take ____ active part in the discussions and show your own opinions about the matters, and at the same time, learn to listen to other people's views ____ (careful). Second, you should learn about your classmates' likes and dislikes, ____ can make you get familiar with them. Only in this way can you get to know more ____ (difference) people and show them that you are just as friendly. I'm always here ready to help. We can take part in some ____ (activity) together and I can introduce you to others.

    There's no need ____ (worry) too much. People will know you more very soon and will like to make friends ____ you if you can do as the above. Just remember, “God helps those who help themselves.”

    Have you ever lost your direction? Then you might find satellite navigation(satnav) systems(卫星定位系统) helpful.

    A satnav system uses groups of satellites. They send information to a receiver(接收器), such as your smartphone. Then they can find where you are.

The earliest satnav system is the Global Positioning System(GPS) of the United States. The US has put 24 satellites into space to make sure GPS can locate(定位) a person correctly and globally. Likewise, Russia's GLONASS and European Union's Galileo can also work globally.

    But China is catching up, building its own BeiDou Navigation Satellite System. BeiDou is China's homegrown satnav system, reported Xinhua. It was set up in 2000, when China sent BeiDou's first navigation experiment satellite into space. With it, China became the third country to build a satnav system after the US and Russia.

    BeiDou can find you wherever you are. It helps you go faster by finding shortcuts(捷径) and using correct directions. When you are in trouble and call the police, they will know exactly where you are if you have a cellphone that uses BeiDou.

    Today, there are more than 20 BeiDou satellites above our heads. And the system is able to offer open services to the Asia-Pacific region(亚太地区).

    In November, China plans to send more BeiDou-3 satellites into space. BeiDou-3's position accuracy(准确度) is less than five meters, Ran Chengqi, a senior designer of the satellite navigation system, told China Daily.

    By 2020, BeiDou is planned to have 35 satellites and begin serving globally, reported Xinhua.