题干

某同学设计了一个用打点计时器“验证动量守恒”的实验:在小车A的前端粘有橡皮泥,推动小车A使之作匀速运动.然后与原来静止在前方的小车B相碰并粘合在一体,继续作匀速运动.他设计的具体装置如图1所示,在小车A后连着纸带,电磁打点计时器电源频率为50Hz.长木板下垫着小木片用以平衡摩擦力.

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阅读下面的短文,然后根据短文内容,在每小题所给的四个选项中选出最佳选项。
                                                                                          C
  “How are you?” is a nice question. It's a friendly way that people in the USA greet each other. But “How are you” is also a very unusual question. It's a question that often doesn't need an answer. The person who asks “How are you?” hopes to hear the answer “ Fine”, even if the person's friend isn't fine. The reason is that “How are you?” isn't really a question and “Fine” isn't really an answer. They are simply other ways of saying “Hello” or “Hi”.
    Sometimes, people also don't say exactly what they mean. For example, when someone asks “Do you agree?” the other person might think, “No, I disagree. I think you’re wrong…” But it isn't very polite to disagree so strongly, so the other person might say “I'm not sure.” It's a nicer way to say that you don't agree with someone.
    People also don't say exactly what they are thinking when they finish talking with other people. For example, may talks over the phone finish when one person says “I've to go now.” Often, the person who wants to hang up gives an excuse “Someone's at the door.” “Something is burning on the stove.” The excuses might be real, or not. Perhaps the person who wants to hang up simple doesn't want to talk any more, but it isn't polite to say that. The excuse is more polite, and it doesn't hurt the other person.
    Whether they are greeting each other, talking about an idea, or finishing a talk, people don't say exactly that they are thinking. It's an important way that people try to be nice to each other, and it's part of the rule of the game of language.