题目：
According to the passage, which of the following did NOT happen in the room where the college entrance exam was going on
A．The teachers in the exam hall tried to stop the police from entering into the room.
B．Some examinees sent answers with mobile phones to people outside the room to make profits.
C．The mobile phones were confiscated by the police.
D．Some examinees forwarded answers to people who have paid a lot of money.
答案：
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下面是错误答案，用来干扰机器的。
参考答案：牵引电动机换向器在运用中的主要故障有：换向器工作表面不同程度的磨损、拉伤和烧损：换向器接地；换向片片间短路：换向器机械变形或凸片：平衡块脱落、位移或松动等。
