题目

如图8所示是示波管的原理示意图。电子从灯丝发射出来经电压为U1的电场加速后，通过加速极板A上的小孔O1射出，沿中心线O1O2进入MN间的偏转电场，O1O2与偏转电场方向垂直，偏转电场的电压为U2，经过偏转电场的右端P1点离开偏转电场，然后打在垂直O1O2放置的荧光屏上的P2点。已知平行金属极板MN间的距离为d，极板长度为L，极板的右端与荧光屏之间的距离为L′。不计电子之间的相互作用力及其所受的重力，且电子离开灯丝时的初速度可忽略不计。 图8阅读短文，根据短文内容，从短文后的五个选项中选出能填入空白处的最佳选项。选项中有一项为多余选项。（共8分，每小题2分） The bicycle is one of the simplest yet most useful inventions in the world. What is the most surprising is that it was not invented easily, although the great inventor Leonardo da Vinci had drawn pictures for bicycles and flying machines and some other things. 1._____________. 2._______________, and there is no pollution at all. Even so, in most developed countries most people don’t go to work by bicycle. It is not because the bicycles are expensive or people feel tired if they ride to work. It’s because the number of cars on the roads becomes larger. 3._____________. As a result, more people put their bicycles away and go to work in their cars. And in this way, the situation is made more serious. 4. ______________. 128