设向量$\overrightarrow{a}$，$\overrightarrow{b}$满足：|$\overrightarrow{a}$|=3，|$\overrightarrow{b}$|=4，$\overrightarrow{a}$•$\overrightarrow{b}$=0．以$\overrightarrow{a}$，$\overrightarrow{b}$，$\overrightarrow{a}$﹣$\overrightarrow{b}$的模为边长构成三角形，则它的边与半径为1的圆的公共点个数最多为（　　）

①加入稍过量的Na₂CO₃溶液；

②加入稍过量的NaOH溶液；

③加入稍过量的BaCl₂溶液；

④滴入稀盐酸至无气泡产生；

⑤过滤．

增加：在缺词处加一个漏词符号(/\)，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： 在错的词下画一横线，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。

    As the saying goes, many little drop of water make an ocean. A small action may lead to major changes took place in society. My friends and I had participated in a voluntary activity at a nursing home last week. We first did some cleaning or washing. After that we spent half an hour chatted with the old people living there. Though it was a tired day, we were happy with that we did. I felt a sense of responsibility by visiting the old. If all of us lend a helping hand to the people in need, they can live in harmony with each other. A society is just as a family. If we are willing to help others, our small actions can make big difference.

读东西半球示意图，回答下列问题．

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