关于电磁场的理论，下列说法正确的是（） A．在电场的周围_爱下问搜题

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问题选择题

关于电磁场的理论，下列说法正确的是（　　）

A．在电场的周围一定存在着由该电场产生的磁场

B．非均匀变化的电场产生的磁场一定是均匀变化的

C．均匀变化的磁场一定产生变化的电场

D．周期性变化的电场一定产生同周期变化的磁场

答案

A、在电场的周围不一定存在着由该电场产生的磁场，原因是若变化的电场就一定产生磁场，若是稳定的电场则不会产生磁场的．故A不正确；

B、非均匀变化的电场产生非均匀变化的磁场，而均匀变化的电场产生稳定的磁场．故B不正确；

C、均匀变化的磁场一定产生稳定的电场，而非均匀变化的电场产生非均匀变化的磁场．故C不正确；

D、周期性变化的电场一定产生同周期变化的磁场，故D正确；

故选：D

完形填空

After the earthquake hit northeast Japan on March 11, 2011, many touching stories that I saw with my own eyes happened around me.
I had to walk home since all the 36 had stopped after the quake. On my way home I 37 an old Japanese lady at the bakery shop who was giving out free bread, which made my heart 38 . A middle-aged Japanese man was holding a sign that said, “Please use our  39 .” He was 40 his house for people to go to the restroom. My friend wanted to  41 others. He stood in the cold with a sign “If you are okay with a motorcycle, I will  42 you to your house.” And then I saw him take one gentleman home, all the way to faraway place!        Then the next day I drove to  43   my car with gas. There was a  44 of gas now and many gasoline stations were either closed or had very  45   lines. I got  46 ,since I was behind 15 cars. Finally, when it was my 47 , the man smiled and said, “48 this situation, we are only giving $ 30 worth gas per person. Is that alright?”“Of course, I’m just glad that we are all able to 49 ,” I said. His smile gave me so much  50 .
I saw a man at the evacuation(疏散) center 51 when people brought food to him, It was the first time in three days that food has been brought to their center. However, after he wiped the tears, his next  52 surprised me. “I am very 53 that we are provided with food. But people in the city next to us haven’t  54 any food at all. Please go to that centre as well.” And when hearing that, I realized there is a bright future on the other side of this 55 .

C．communication

A．advertising

B．Together with

D．entertainment

填空题

根据设备缺陷对安全运行的影响程度，设备缺陷分为（）、（）、（）三类。