下列关于曲柄压力机的叙述错误的是（）。
A．压力机的工作原理：它由电动机通过皮带轮及齿轮驱动曲轴转动，曲轴的轴心线与其上的曲柄心线偏移一个偏心距，从而便可通过连杆(连接曲柄和滑块的零件)带动滑块做上下往复运动
B．压力机的受力系统：冲压件的变形阻力全部传递到设备的机身上，形成一个封闭的受力系统。压力机运行时，除本身重量对地基产生压力外，还包括传动系统的不平衡对地基的振动造成的压力
C．压力机运动分析：曲柄滑动机构的滑块运动速度随曲柄转角的位置变化而变化，其加速度也随着做周期性变化
D．对于节点正置的曲柄滑动机构，当曲柄处于上死点(a=00)和下死点(a=1800)位置时，滑块运动速度为零，加速度最大
参考答案：B
| 阅读理解。 |
| Do you have things that your family no longer uses? Things like old toys, clothes or TVs? In Britain, people collect things and sell them in a garage or yard. People often hold these sales at the weekend. They put up an ad before the day. Then on the day they put out all the old things in front of their house. Usually these things are cheap. These sales are a good way for people to clean up their houses. Instead of just throwing things away, people can make a little bit of money from them. Sometimes if the things are very old, people will give them to charity. Some kids sell their old toys. Perhaps they played a game many times and now it bores them. With the money they can buy a new toy! |
| 1. In which country do people have yard sales? |
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| A. China. B. The United Kingdom. C. The United States. D. France. |
| 2. When do they usually have the yard sale? |
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| A. At work. B. On weekdays. C. After school. D. On the weekend. |
| 3. The writer thinks it's _____ to have yard sales. |
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| A. good B. expensive C. difficult D. strange |
| 4. What do some people in Britain do with things that are too old? |
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| A. They sell them. B. They collect them. C. They give them to charity. D. They throw them away. |
| 5. Which is true according to the passage? |
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| A. In Britain, people often sell old things in the street. B. Selling old things is a good way for people to clean up their houses. C. People can't make any money from selling old things. D. When people sell old things, they don't put up ads. |