某地发生民工中毒，为误食亚硝酸钠（NaNO₂）所致，亚硝酸钠中氮元素的化合价是（  ）  

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：在错的词下划一横线，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。
    Last summer I go to America and studied at a language school. I had many wonderful experience, but I also had a sad one. One day, the school held party , where I invited to talk about Chongqing. After that they asked me a lot of things about China. But I couldn't explain them with English clearly. I felt sadly. I learned a lesson from this experience. I have already studied English for eight years, I can't use it very good. I must work hard to improve my spoken English so that I will not be able to communicate freely with foreigners. I hope I can be a bridge between China and others countries in the future.

关于生物对环境的适应叙述错误的是（　　）

如下图所示，小刚早晨7时出门步行到离家6千米远的车站去。返回时骑自行车，上午9时到达家中.

干燥时节，两个同时用毛皮摩擦过的气球互相靠近时会出现排斥现象。