在动摩擦因数μ=0.2的粗糙绝缘足够长的水平槽中，长为2L的绝缘轻质细杆两端各连接一个质量均为m的带电小球A和B，如图为俯视图（槽两侧光滑）．A球的电荷量为+2q，B球的电荷量为﹣3q（均可视为质点，也不考虑两者间相互作用的库仑力）．现让A球处于如图所示的有界匀强电场区域MPQN内，已知虚线MP恰位于细杆的中垂线，MP和NQ的距离为3L，匀强电场的场强大小为E= $\frac{1.2mg}{q}$ ，方向水平向右．释放带电系统，让A、B从静止开始运动（忽略小球运动中所产生的磁场造成的影响，重力加速度为g）．求：

假定英语课上老师要求同桌之间交换修改作文, 请你修改你同桌写的以下作文。文中共有10处语言错误,每句中最多有两处。每处错误仅涉及一个单词的增加、删除或修改。

增加: 在缺词处加一个漏字符号(∧),并在其下面写出该加的词。

删除: 把多余的词用斜线( \ )划掉。

修改: 在错词下划一横线, 并在该词下面写出修改后的词。

注意: 1）每处错误及其修改均仅限一词；

2）只允许修改10处, 多者(从第11处起)不计分。

Dear John,

    I'm delighted that you are coming to Beijing this summer vacation. You asked many question in your letter, and now let me answer them.

    Beijing, the capital of China, is attractive city with a long history. You can found all kinds of delicious food in Beijing, of that the most famous is the roast Beijing duck. I hope we will enjoy it. Besides, there is a great number of tourist attractions worth visiting, such as the Great Wall, the Summer Palace or the Forbidden City. These scenic spots have attracted millions of tourists in this year. I promise you will have a wonderfully time here in Beijing.

    I'm looking forward to hear from you as soon as possible.

Yours,

LiMing