电视中围棋评论员讲评棋局时，所用的棋子是磁铁块，棋盘是磁性材料制成的，棋子可以静止在竖直放置的棋盘上．下列说法正确的是（ ）
A．棋盘对棋子的吸引力大于棋子对棋盘的吸引力
B．棋盘对棋子的吸引力与棋子对棋盘的吸引力方向相同
C．棋子受到的摩擦力大于棋子的重力
D．棋子受到的摩擦力与棋子的重力大小相等
A、棋盘对棋子的吸引力与棋子对棋盘的吸引力是作用力与反作用力，大小相等，故A错误；
B、棋盘对棋子的吸引力与棋子对棋盘的吸引力方向相反，故B错误；
C、棋子受到竖直向下的重力，向上的摩擦力而静止，处于平衡状态，棋子受到的摩擦力等于棋子的重力，故C错误；
D、棋子受到的摩擦力与棋子的重力是一对平衡力，大小相等，故D正确；
故选D．
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| Many people use e-mails to communicate with each other. If you can surf the Internet, it is a great way to 1 messages easily, quickly and as often as you like. It is not necessary to find a 2 or paper, and you don't need an envelope or a stamp. Are we missing 3 if we don't write paper letters to people? Most people would probably answer, "No, it is a 4 of paper anyway, and this is a much better way. "However, I would like to 5 . I believe that this kind of letter people used to write was probably more important than billions of e-mails sent today. Of course, if people need to send 6 messages, e-mail is the best. If time is important, e-mail is 7 . However, if you want really to get in touch with someone, a letter is more 8 . Though I'm a teenager, I have to say that I feel very 9 to receive a letter in the mail. I love seeing my name written by hand on an 10 " You make yourself a cup of coffee, sit down in a comfortable place, open your letter, and enjoy everything that the person puts into the letter for you. What a gift! | |||
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