为了保证偏转线圈使电子束作重复性摆动和偏移，偏转线圈的磁芯可选用高频特性好的材料、在偏转频率高于10kHz时，应采用( )偏转线圈。
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What does the Writer think about machines with human - like ability
A. He believes they will be useful to human beings.
B. He believes that they will control us in the future.
C. He is not quite sure in what way they may influence us.
D. He doesn't consider the construction of such machines possible.
Which of the following is NOT MENTIONED in the passage（）
A. Gunpowder.
B. Needles.
C. Paddle boats.
D. Bridge design.
《合同法》最基本的原则是( )
A. 合法原则
B. 合同自由原则
C. 诚实信用原则
D. 鼓励交易原则
电子束焊机应有良好的地线装置，进行绝缘试验时，试验电压应为额定电压的( )倍。
A. 1
B. 1.2
C. 1.4
D. 1.5
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