My parents want to take me to visit Beijing this summer holiday. I want to plan a good trip. So I must learn something about Beijing on the Internet. Beijing is a beautiful city. Autumn is the best season. There are many places of interest in Beijing. We want to visit the Great Wall, the Palace Museum and the Summer Palace. Beijing Duck is very famous all over the world. I hope I could have a try.

    We can get to Beijing by train. I want to buy a gift for my good friend. I also want to send a postcard to my teacher. I think I will have a great holiday.

人体的每个____都有各自的功能，都需要我们____。

 如图所示，闭合开关后，关于螺线管两端的磁极，判断正确的是（　　）

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