There was a pet store and the owner had a parrot. One day a 1 walked in and the parrot said to the man ,“Hey you!” The man said, “What!?” The parrot said, “Your 2 is really ugly.” The man got very 3 and went to the store owner and said, “Your bird just  4 my wife. It said she was ugly.”

    The owner stormed over, 5 the bird, took it into the “black room,” shook it a bit, 6 out a few feathers, and said, “Don't ever, ever say anything to  7 my customers again. You got that！！！”

    With that 8 he took the bird and put it back into its cage. The old bird shook out its    9  and relaxed in its cage. A couple of weeks 10 and in walked this guy and his wife again. The parrot said, “Hey you!” The guy said, “What!?” The parrot answered, “You know that.”

质量为m、长度为L的通电技术杆ab水平搁置在两压力传感器上（杆与传感器绝缘）．处于磁感应强度为B，垂直纸面向里的匀强磁场中，金属与磁场垂直．如图所示．金属杆中的电流大小和方向均可以改变，但金属杆一直处于静止状态，两压力传感器度数时刻保持一致，重力加速度为g，则：

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①实践是获得认识的唯一途径     
②实践是认识的基础

③间接经验最终来源于实践       
④实践是认识的产物